Here's what a Christian would say: The essence of sin is the refusal to submit to God as supreme master. It happens that He invented sex for purposes of His own (one of which is our enjoyment, btw), and to accomplish those purposes, He has set down some rules for it.
What we call the left is just a political expression of the rejection of God as Master. Leftists consider that they have the right to make all their own rules. Their ideal is absolute personal autonomy, but that is not possible in a world with more than one person in it. The minute you put two people within contact distance, each is curbed to some extent by the other. It turns out that the left is willing to accept practically any sort of limitation in tradeoff as long as it leaves them free to chase what is for them The Big One: absolute sexual freedom. You can tax them, regulate them, herd them here and there, but don't tell them who they can't take to bed. Especially, you don't remind them that Somebody Whom they would rather ignore has something to say about it.
You can leave Christianity out of it completely. Homosexual behavior is in complete defiance of natures intended design, period. It’s an unnatural act, unnatural behavior, self destructive, and destructive to the species in general.
nature guards against these kinds of abominations, which is why diseases like aids, rectal cancers etc. crop up. Left to run it’s course, nature will restore it’s natural order, and dictate natural behavior.
Animals are even smart enough to instinctively obey natures laws, Humans for some reason can’t. They can and do actually, but perversion and selfish desire, political correctness and greed for power overrides natural instinct and common sense natural laws.