Sin is an arbitrary concept. Homosexuality is not illegal any more (in most Western countries), nor is it defined as a disease.
It's not an identity, it's just a sexual orientation, just like heterosexuality is.
It's more "normalized" to be homosexual than it is to be American. About 5-10 percent of the population are homosexual, about 4 percent are American.
The definition of marriage have been changed many times. Changing the text in the law from "the man and wife" to "the couple" should hardly rock the world. And, lo and behold, it didn't. The Netherlands have had their gender-neutral marriage laws for over 15 years now, and they seem to be doing fine.
Besides, gays and lesbians don't sleep with mankind as with womankind, they sleep with human beings as individuals.