I don't think I ever had a female TA as an undergrad (and only one prof, but she was 60+). Tech school
Now that I'm a TA, I've had a couple of little crushes on students in my sections. I've never done anything about it though. I don't want to get hit with sexual harassment charges or whatever. Otherwise I'd definitely be up for it. I'd give their papers to someone else to grade so there wouldn't be any "ethical problem."