Two of my friends (who are girls) asked me this once... it was quite an awkward conversation.
But it's such an interesting question. It's like it's hardwired into our heads - it's crazy. It's obviously closely linked with biology, but I think the people who say it's cos we 'sucked our mum's tits' as a baby is retarded. Why then, don't gay guys go bonkers for boobies?
Mmm, boobies.
Oh and I think Jerry Seinfield once questioned this, and said it's the fact that they're covered up... and if girls were to cover up their arms, guys would go ballistic for that hot forearm.
But I'unno.