One of the standard conventions is for the uke to protest and resist, and for the seme to ignore it.

Actually, I have read quite a few fics in Japanese where the seme doesn't just ignore protests, he (we'll assume it's a guy) goes, hey, that's a turn-on, and then tells the uke it's his/her fault for tempting him. And, yeah, you know the rest. I have to say that out of all these little rituals I see in doujinshi style anything, this is one of the ones that I still just can't accept as sexy. Drives me effing insane.

I think the reason doujinshi style gets away with having "uke" and "seme" being immutable roles is because it's a style that comes from pictures. If you can draw the character in question, it's much easier to inflict these roles on him or her because they're already something you can recognize. The drawing itself, the appearance, is a huge part of characterization. (By the way, I'm PRETTY SURE I stole this idea from reading one of those essays on aestheticism.com that that cool lady writes. I forget her name, but yeah, she smart. You're all so smart. Gumdangit.)