I don't know about sex slaves. I would guess 1 out of 4 really don't want to be there.
In reality any wife who isn't physically attracted to her husband could be considered a sex slave.
The bottom line is that in all male - female interactions every woman's motivation varies. Some do it because they love it (and we are lucky to know them) others because they think they have to or because they use it to get what they want (money, a big house, kids, etc.) then there are the ones who would really rather not.
I didn't mean to imply that TP kidnaps asian woman off the streets in the East and keeps them handcuffed to the radiator when they aren't servicing us Barbarians. It's just that if you are more than a heat seeking missile you can pick up on the vibe from a woman even if she is an inscrutable oriental.