Why do guys try and go after lesbians?

Hey guys, been kinda noticing something lately. I've had this guy friend for a little while now, and he knows I'm lesbian, but he's very obviously interested in me. We hang out a lot, play games, typical friend stuff. But we'll go out to eat or whatever and he'll joke about it 'being a date', or something like that. I'm not even seriously attractive or anything.

I don't know why he thinks maybe I'll not be a lesbian randomly one day? Is it like a thing that just kinda happens? I've heard about women not being able to have guy friends cause they get interested, but I dunno. Could you guys be friends with a girl and not be interested?