Is there only a single sex for each class?

Yes.

If we had unlimited resources it's definitely not a decision we'd make. But, having an extra gender option available for each class would require skinning all the armour models twice, or in some case creating separate models for more revealing armour like some of the primalist's chest pieces. Additionally there'd be a lot of extra animation work, making sure all the animations work for both, or in some case creating additional animations where they don't. Getting the quality and diversity of armour visual models to where it is at 1.0 and getting animations to their standard at 1.0 has been a very significant task for a team of our size, and having to account for twice as many character models would have slowed that down a lot.