[creekster]

Not to put too fine a point on this scintillating debate, but what are you all talking about? really, it is not clear. Are we talking the morality of various gender pairs based on Aristotelian ethics, or some other humanist code? If we are tlaking about the LDS code, then it is really rather straightforward: it is sinful becasue the prophet tells us unambiguously that it is. It is against God's will, therefore it is a sin.

Now if you want to talk about other codes of conduct or social policies you might reach a very different answer. So that term needs to be defined before this discussion can make too much sense.