"Turretinfan, you have espoused the view that for an act to be considered immoral, we must find the condemnation of such an act in Scripture."
This seems like an accurate description of my position, although (as discussed below) certain implications you have drawn from this are not correct.
"Scripture is not only your rule of faith regarding theological positions, but moral ones as well."
The moral law is one important branch of theology. We say that the Scriptures of the Old and New Testaments are our supreme (and the only infallible) rule of faith and life. We see this same sentiment in the fathers as well, especially those who preceded the scholastics.
Scripture itself teaches this:
Deuteronomy 8:3 And he humbled thee, and suffered thee to hunger, and fed thee with manna, which thou knewest not, neither did thy fathers know; that he might make thee know that man doth not live by bread only, but by every word that proceedeth out of the mouth of the LORD doth man live.
Matthew 4:4 But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God.
Luke 4:4 And Jesus answered him, saying, It is written, That man shall not live by bread alone, but by every word of God.
Deuteronomy 5:33 Ye shall walk in all the ways which the LORD your God hath commanded you, that ye may live, and that it may be well with you, and that ye may prolong your days in the land which ye shall possess.
Proverbs 4:4 He taught me also, and said unto me, Let thine heart retain my words: keep my commandments, and live.
Psalm 37:23 The steps of a good man are ordered by the LORD: and he delighteth in his way.
Proverbs 2:20 That thou mayest walk in the way of good men, and keep the paths of the righteous.
Psalm 143:8 Cause me to hear thy lovingkindness in the morning; for in thee do I trust: cause me to know the way wherein I should walk; for I lift up my soul unto thee.
The Westminster Confession puts it this way:
Under the name of holy Scripture, or the Word of God written, are now contained all the Books of the Old and New Testament, which are these: [list of 66 book canon is presented, but I have omitted it] All which are given by inspiration of God, to be the rule of faith and life.- Westminster Confession of Faith, Chapter 1, Section 2
Augustine similarly speaks of the rule of life (vitae regulam).
Augustine (writing in A.D. 387):
The man, then, who is temperate in such mortal and transient things has his rule of life confirmed by both Testaments, that he should love none of these things, nor think them desirable for their own sakes, but should use them as far as is required for the purposes andduties of life, with the moderation of an employer instead of the ardor of a lover.Latin text:
Habet igitur vir temperans in huiuscemodi rebus mortalibus et fluentibus vitae regulam utroque Testamento firmatam, ut eorum nihil diligat, nihil per se appetendum putet, sed ad vitae huius atque officiorum necessitatem quantum sat est usurpet utentis modestia, non amantis affectu.Citation: Augustine, On the Morals of the Church and the Morals of the Manichaeans, Two Books, Book 1 (On the morals of the Church), Chapter 21 (Section 39)
Augustine (writing in A.D. 387):
What of justice that pertains to God? As the Lord says, "You cannot serve two masters," [Matthew 6:24] and the apostle denounces those who serve the creature rather than the Creator, [Romans 1:25] was it not said before in the Old Testament, "You shall worship the Lord your God, and Him only shall you serve?" [Deuteronomy 6:13] I need say no more on this, for these books are full of such passages. The lover, then, whom we are describing, will get from justice this rule of life, that he must with perfect readiness serve the God whom he loves, the highest good, the highest wisdom, the highest peace; and as regards all other things, must either rule them as subject to himself, or treat them with a view to their subjection. This rule of life, is, as we have shown,confirmed by the authority of both Testaments.Latin text:
Quid de iustitia quae ad Deum pertinet? Nonne cum et Dominus dicat: Non potestis duobus dominis servire [Matthew 6:24], et Apostolus redarguat eos qui creaturae potius quam Creatori [Romans 1:25] serviunt, in Veteri Testamento prius dictum est: Dominum Deum tuum adorabis, et illi soli servies [Deuteronomy 6:13]? Sed quid opus est hinc plura dicere, cum sententiis talibus ibi plena sint omnia? Hanc ergo iustitia vitae regulam dabit huic amatori de quo sermo est, ut Deo quem diligit, id est summo bono, summae sapientiae, summae paci libentissime serviat ceteraque omnia partim subiecta sibi regat, partim subicienda praesumat. Quae norma vivendi, ut docuimus, utriusque Testamenti auctoritate roboratur.Citation: Augustine, On the Morals of the Church and the Morals of the Manichaeans, Two Books, Book 1 (On the morals of the Church), Chapter 24 (Section 44)
Augustine (writing in A.D. 387):
This discipline, then, which is the medicine of the mind, as far as we can gather from the sacred Scriptures, includes two things, restraint and instruction. Restraint implies fear, and instruction love, in the person benefited by the discipline; for in the giver of the benefit there is the love without the fear. In both of these God Himself, by whose goodness and mercy it is that we are anything, has given us in the two Testaments a rule of discipline. For though both are found in both Testaments, still fear is prominent in the Old, and love in the New; which the apostle calls bondage in the one, and liberty in the other. Of the marvellous order and divine harmony of these Testaments it would take long to speak, and many pious and learned men have discoursed on it. The theme demands many books to set it forth and explain it as far as is possible for man. He, then, who loves his neighbor endeavors all he can to procure his safety in body and in soul, making the health of the mind the standard in his treatment of the body. And as regards the mind, his endeavors are in this order, that he should first fear and then love God. This is true excellence of conduct, and thus the knowledge of the truth is acquired which we are ever in the pursuit of.Latin text:
Haec tamen disciplina de qua nunc agimus, quae animi medicina est, quantum Scripturis ipsis divinis colligi licet, in duo distribuitur, coercitionem et instructionem. Coercitio timore, instructio vero amore perficitur eius dico cui per disciplinam subvenitur, nam qui subvenit, nihil horum duorum habet nisi amare. In his duobus Deus ipse cuius bonitate atque clementia fit omnino ut aliquid simus duobus Testamentis, Veteri et Novo, disciplinae nobis regulam dedit. Quamquam enim utrumque in utroque sit, praevalet tamen in Veteri timor, amor in Novo; quae ibi servitus hic libertas ab Apostolis praedicatur. De quorum Testamentorum admirabili quodam ordine divinoque concentu longissimum est dicere et multi religiosi doctique dixerunt. Multos libros res ista flagitat, ut pro merito, quantum ab homine potest, explicari et praedicari queat. Qui ergo diligit proximum, agit quantum potest ut salvus corpore salvusque animo sit, sed cura corporis ad sanitatem animi referenda est. Agit ergo his gradibus, quod ad animum pertinet, ut primo timeat deinde diligat Deum. Hi mores sunt optimi, per quos nobis etiam ipsa provenit, ad quam omni studio rapimur, agnitio veritatis.Citation: Augustine, On the Morals of the Church and the Morals of the Manichaeans, Two Books, Book 1 (On the morals of the Church), Chapter 28 (Section 56)
Augustine (writing in A.D. 387):
But why say more on this? For who but sees that men who dare to speak thus against the Christian Scriptures, though they may not be what they are suspected of being, are at least no Christians? For to Christians this rule of life is given, that we should love the Lord Our God with all the heart, with all the soul, and with all the mind, and our neighbor as ourselves; for on these two commandments hang all the law and the prophets.Latin text:
Sed quid hinc plura? Quis enim non videat eos qui contra Scripturas christianas haec audent dicere, ut illud non sint quod homines suspicantur, certe tamen non esse christianos? Nam christianis haec data est forma vivendi, ut diligamus Dominum Deum nostrum ex toto corde, ex tota anima, ex tota mente 91, deinde proximum nostrum tamquam nosmetipsos 92. In his enim duobus praeceptis tota lex pendet, et omnes prophetae 93.Citation: Augustine, On the Morals of the Church and the Morals of the Manichaeans, Two Books, Book 1 (On the morals of the Church), Chapter 30 (Section 62)
"Therefore, the God given faculty of rational thought which separates man from beast cannot in any way condemn an act as immoral unless it has scriptural warrant to do so."
This does not follow. One's innate knowledge of God's law may lead one to condemn certain things as immoral without being taught from Scripture. However, of course, all those who understand original sin must also see the danger of treating one's conscience as though it were infallible. One conscience is, therefore, a bound on what one is permitted to do, but it does not serve as a rule by which we are to condemn others. To condemn others, we need a higher authority than our own conscience.
"Is this your position?"
Not quite. See the distinctions above.
"Why is homosexuality immoral?"
Why it is wrong may be different from how we know it is wrong. It is wrong because it contrary to the moral law of God. Whether God's nature necessitated that or whether it was a voluntary law is an interesting question that's not really germane to our discussion.
We know it is wrong both from Scripture and (for many of us) from conscience.
"Is it simply because the Bible condemns it as such?"
See above. We know it is wrong from the Bible. The reason that it is wrong is the moral law, which is revealed to us clearly through the Bible and less clearly through the light of nature.
"If the Bible were silent on the issue of homosexuality, would it have been moral to engage in it?"
It wasn't moral prior to Scripture being written. Scripture reveals God's law to us - it is not itself the basis of morality. Rather Scripture is the revelation of God's law. The Bible would have been silent on the issue of homosexuality, if it were a matter of indifference. It speaks against the sin because one of the purposes of Scripture is to show us the way we ought to live.
"Why has God condemned homosexuality; is this something that He has communicated to us?"
We might argue over whether God has communicated the reason for his condemnation clearly. It should be apparent that God created Eve (not Steve) for Adam. Consequently, we might reasonably infer that one reason for the prohibition on homosexuality is is contrariety to the Creation ordinance of marriage. This looks like a voluntary law (as opposed to a natural law), but again whether it is or not is not really germane to this discussion.
"What are the inherent principles involved?"
I don't know what this comment refers to.
"My position is that the natural law is the rational agent’s participation in the eternal law."
It looks like the commenter's position is borrowed from Aquinas: "It is therefore evident that the natural law is nothing else than the rational creature's participation of the eternal law." (Summa Theologica, 1st part of the 2nd part, Question 91, Article 2)(link)
I do not know whether the commenter would also agree with Aquinas:
Article 6. Whether the law of nature can be abolished from the heart of man?- Thomas Aquinas, Summa Theologica, 1st part of the 2nd part, Question 94, Article 6 (link)
I answer that, As stated above (4,5), there belong to the natural law, first, certain most general precepts, that are known to all; and secondly, certain secondary and more detailed precepts, which are, as it were, conclusions following closely from first principles. As to those general principles, the natural law, in the abstract, can nowise be blotted out from men's hearts. But it is blotted out in the case of a particular action, in so far as reason is hindered from applying the general principle to a particular point of practice, on account of concupiscence or some other passion, as stated above (Question 77, Article 2). But as to the other, i.e. the secondary precepts, the natural law can be blotted out from the human heart, either by evil persuasions, just as in speculative matters errors occur in respect of necessary conclusions; or by vicious customs and corrupt habits, as among some men, theft, and even unnatural vices, as the Apostle states (Romans 1), were not esteemed sinful.
Such an admission tends to undermine the use of "natural law" standing alone as a rule for others, even if it is an individual's participation in the eternal law (whatever that is supposed to mean). The light of nature leaves the individual without excuse, but it can be obliterated (variously) as to many details, and consequently is not an infallible authority from which to build a system of morality by which we condemn others.
"Please explain your position."
Hopefully the explanation above suffices.