heh, I don't think a man (or woman, probably) can just put his sexual drive aside when he chooses to. So in short, being "only friends" with a hot female isn't very likely. However, I don't think that the man will automatically actually think that they're going to have sex, I think it can stay on a "flirting level" and that does no harm. I think small, innocent flirting is there to allow us to test if we're still attractive in the opposite sex in ways that won't harm our possible ongoing relationships with someone else. Unless that life companion is very jealous.
Bleh, short version: I don't think one can turn off his penis even if he wanted to, but that doesn't mean he has to actually try and f*ck every woman he spends time with.