Principle, please

In the past couple of weeks I've engaged in some interesting discussions with progressive Christians (including Catholics) on the topic of same-sex marriage and the morality of homosexual acts, one discussion occurring in the comments of this post at The Right Christians, the other in the comments of this post at Matt Zemek's Wellstone Cornerstone.

Beside making positive rational and non-religious arguments defending the notion that same sex marriages should not be legalized and that homosexual acts are immoral (on the former see especially this article by Princeton political philosopher Robert George), I think it's also beneficial to ask Christians (and others) who support the legalization of same sex marriages (sic) to articulate the principle by which they view homosexuality as moral while rejecting (any) other form of consensual sexual behavior as immoral. I pushed Matt especially on this point (in the second of the links above). Basically, I don't think that such a principle exists... if homosexual acts are moral, what moral or ethical principle prohibits any other form of consensual sexual behavior, regardless of the gender or number of the participants?