But Diablo is male, any dialogue from that point still calls him a male and their was only 1 female in the 7 which was Andariel......which is backed up when you pick up the Andariel lore scroll in Act 2 and Deckard says "she was the only female evil".
Yes Diablo as The Prime Evil has an awful lot of junk in that trunk (check ze sexy walk in the Act 4 vid) whilst being stacked to boot but he's still a male.
/edit:
The Fall of the High Heavens, The Light of Hope and Beneath the Silver Spire cinematics all refer to Diablo as a he (at least the Barb ones do) and are all post transformation into the Prime Evil. So ye regardless of boobage and booty it's a dude.