My g/f is usually fairly quite and shy. however, when having sex she gets very vocal and her language gets quite crude.
She obviously enjoys that, so I encouraged her more. Well, the next thing I know, while having sex, she is telling me how much she wants to be in a 3-some, either with another guy or girl, and exactly what she wants them to do with her and vice-versa. Actually it is quite a turn-on for me too, to hear her.
Afterwards she says it is only fantasy and I beleive her. But when i asked her if she wanted me to also fantasize aloud the same way, she said no, that didn't turn her on. But she couldn't say why.
Anyone have any ideas on this? 
To be psychoanalytic, she is using a fantasized person to address in telling you what to do. Guys should be so lucky.
Oh, I'm not compalining, beleive me!
However, I do wonder why she is not turned on by the thought of me "returning the favor", even if I am saying similar things to her. Any thoughts on that part?
it's a girl thing. She won't get turned on by you doing it because of the thought of her man with another women. If your not happy with her not allowing just ask her to stop. She may see that she's doing the samething and allow you to try the same.