I've never understood that whole thing... why you can't still be friends after a breakup. But sometimes I attribute that to getting too physical when the relationship isn't serious enough to warrant it. I think people actually fall in love more and more when making out/having sex, and it hurts worse when you break up. I say go for it if you like the person that way, but really hold back on the sex until there's an appropriate commitment level. Then if you break up, it's not like you've given your all for that person. You could still see them as a friend without it breaking your heart.