It's best if she's the only one he is aware of. The angel would find Damon immediately suspicious.

There is a lot that Castiel isn't saying or may not even be aware of with his condition. There is certainly some evidence to suggest he has wanted sex before as an angel though often when he was weakened or cut off from heaven. This conversation is a first for all that. "I hadn't really thought about it." Maybe that means he does not want to. This line of questioning is very confusing. But if one thing is for sure, he wouldn't return home just to remain an angel because his friends are here.