Ah, Spider Robinson. I remember buying a stack of his books at Borders around age 12 and having the clerk give my mother an alarmed look. Mom just waved her hand ....

I think it's pretty normal for science-fiction-reading middle- to upper-middle-class kids to think that alternative sexuality is "normal" and to feel guilty for being vanilla/monogamous/whatever. (I used to feel a lot of pressure to be polyamorous.) Interestingly, though, there still seems to be a lot of internalized stigma about certain forms of sexuality, as demonstrated for example in my coming-out story. I would imagine that most people here fit that tribal group.

Still, within that tribal group I still encounter a lot of people with assumptions I'd call weird and/or irrational, which is why I asked specifically what kind of sexuality-related arguments would lead you to consider someone to be defending a non-mainstream belief. I think your legalized rape post (it was forwarded to me last year, actually, and I still haven't decided how I feel about it) is a definite example of defending a non-mainstream belief, but I wonder if there are less dramatic ones.