I don't entirely understand your is-ought distiction. Are you asking whether, if we think it is a sin, then why?
Well for one, because Scripture is quite clear about the immorality of homosexual acts, Old and New Testament, and Scripture's word alone should provide the 'why.' Is homosexuality a sin? Yes, Scripture says so. Why? Because Scripture says so. (Many would make an argument that Jesus and Paul had in mind homosexual acts and not homosexuality, since homosexual identity is a purely modern construction; it would agree that homosexuality as an identity is a purely modern construction (I'm a Foucault fan), but I actually thinks it makes the situation worse since it means people are constructing their identity around sin)
Granted, this may seem elementary. But as Christians it should be good enough for us. Then again, it's never good enough for me, so I'll venture a real explanation...
Humans are created in the Image of God. We often use this truth to offer normative declarations on human rights issues and argue for the fundamental dignity of human persons. Yet we often fail to consider what the Image actually is, apart from some abstractions (personal sovereignty, creativity, etc.).
I submit that when questioning the nature of the Image of God, we must take into account the highest truth revealed to us about God- that God, in his very nature, is Triune. God is a self-sustained community.
The Divine Son is the image of the invisible God, we know from Colossians and Hebrews. The Father, being self-aware, has an eternal self-perception of himself that, himself being perfect, is perfect. And since existence is a quality of perfection, this self-image exists- the Divine Son, who exists from all eternity. Then, the Father and the Son also love each other, and this love is also perfect- existing, therefore, as the Holy Spirit. This is the traditional Western understanding of the operation of the Trinity (I have my own additional theories garnered from modern philosophy's insights, but they don't really change the point and are quite technical).
The point being, God is a self-sustained community involving the mutual sharing of one essence between three persons. This sharing of essence, is which each person participates fully, involves an eternal self-oblation (pouring) and sacrificial self-giving of being to each of the other divine persons in love. This is the beautiful heart of our God.
Now the oneness of essence is expressed in the great creed of the Jewish people found in Deut. 6:4: Shema Yisrael Adonai Eloheynu Adonai echad, or, Hear oh Israel, the Lord our God is one. The word here for one/oneness, echad, is actually the same word used in Genesis 2:24 when refering to the result of sexual relations between male and female- the coming together of two as one flesh. And this sharing of being (ontological unity) results in children, just as the love between the Father and the Son results in the Holy Spirit.
The point is this- the Image of God is not simply found in our persons as individuals (personal sovereignty and dignity, creativity, etc., though certainly that), but as a self-giving community found in the traditional family structure.
Note that I am not saying that homosexuals cannot participate in truly loving acts or engage in sustained monogomous relations. I am simply saying that the traditional family structure, based on the monogomous and heterosexual covenant of marriage found in God's original creative act (note: I am not a creationist), is itself the truest expression of the Image of God- and to warp the Image is the essence of sin.
That said, I'm profoundly libertarian on the issue of homosexual couplings in civil society. I don't see where in the US Constitution the federal government has the authority to define marriage, let alone grant tax breaks to married households, nor even why individual state authorities should be allowed to define marriage. I understand why states should be allowed to give longterm tax incentives to domestic partnerships- be they heterosexual married couples, homosexual partners in a self-proclaimed "marriage" relationship, a son and a single mother, or college apartment roomies- since longterm domestic partnerships provide for social stability. But why is someone's sex life the government's business at all?
Homosexuality is a grave sin, but not a crime. The only crime would be for government to definite marriage for the culture, one way or other.
