Does it hurt when girls take it up the ass?

It shouldn't, if you're doing it right! Since the anus does not produce lubricant, you'll need something like KY jelly. (Don't use petroleum-based lubricants with latex condoms!) It may help if the woman straddles the man and lowers herself onto his penis, so she is in control of the speed and depth of penetration. It's absolutely necessary for the woman to be able to relax her sphincter, so she might want to practice with a dildo, etc., first.