Traditional bellydancing was originally performed by (often crossdressed) males. (Yes, I know it's possible that Altair's story takes place way before that type of dancing began to develop, but I'm just handwaving it for the sexy, okay?)

One of the many curious abilities the Brotherhood taught Altair to help him work under cover and infiltrate his targets' inner circles is to act like a professional belly dancer. (Why do you think he sways his ass the way he does when he walks? XD) As a result, he's very proficient at it - elegant, graceful and unbelievably seductive.

One day, he dances for Malik, complete with veils, tiny bells and bared midriff. Tender sex follows. Bonus points if he rides Malik.