Saw a book about this before, it was titled platonic sex. Never
read it though. Maybe you might want to go look for it.
Actually, this kind of things do happen. Two friends, one guy, one
girl, who already know each other, suddenly have sex. It's not that
the girl suddenly strip naked infront of the guy or anything,
sometimes it just happens. On a dark stormy night, one guy and girl
in the room, then things happen. Sometimes it's not even about
anybody seducing anybody, it just happens. Spontanous combustion of
passion?
Haven't actually done 'it' with any of my friends, but there have
been times where I engage in petting with my female friend. Still
friends after that but will feel kind of awkward after that. Best
it's not somebody you see often.