If ZackxCloud then Zack is a bastard. He abandons Cloud for much of their story and doesn't give him the attention he deserves. I've always felt I had a lot in common with Cloud growing up. Both of us had issues with our parents, were isolated as kids; weak, quiet, occasional troublemakers with a crush on someone they could never tell. I, for one, despite not being adept in social situations despise being abandoned. I'd imagine Cloud feels the same.

Aerith's importance was to finally take Cloud's virginity. Yes, his idiotic behavior is due to not having been laid... ever. The closest experiences he's had are semi-homosexual. Of course, in-game dialogue choices may cause Tifa, Yuffie, or Barret to be the one he screws during Gondola ride. x_x