I'm not religious and am 100% in favor of same sex marriage. I'm 100% heterosexual.
This is probably worth its own post, but since it has to do with the current conversation, I'll post it here. I'm reading Frank Barlow's biography on King William II Rufus of England (k. 1087-1100). There's 99.9% certainty that William II was very homosexual. A gallant man's man kind of homosexuality.
Anyway, this is illuminating, from page 108:
Barlow then goes on to quote St. Peter Damian, who evidently argued that gay sex was
worse than bestiality.
Okay, here's how to deal with this issue in the present. Taking those quotes above, we're dealing with an institutional mindset (Catholic or Protestant) that likely misunderstands what sex even is beyond its function. The church's position is all very ludicrous, of course. And it's an opinion that was formed at a time when nothing substantive was known about either psychology or evolution. Look at the amount of contemporary studies that show that a healthy, regular sex life is healthful in all kinds of ways. That these people deny that is nutty.
Now, I don't think the Bible has anything to do at all with us here in the present. So to me the argument that would make them look foolish in demanding celibacy and in demanding only heterosexual marriage would be to quote 1) Psychology and 2) evolution. How can either be countered? Human beings are not above sex and / or sexual impulse, and to deny it is to deny who we are. Nature makes mostly heterosexual people, but it also makes gay people.