she has probably been abused as a child & young women. pedophies train children to do their bidding while they are young & impressionable. this then becomes normal for them. they don't know any other way. also rex has only slapped her a couple of times with others in the room so he knows that it is not acceptable behavier. and yes he almost certainly is treating her much worse at home. i believe that anna has abandonment issues. she may have had a father that left, went to prison, or she never knew him. she holds onto alex because he represents the missing protector that she never had. she's made a bad choice but she doesn't believe that she deserves better. that is why anna & dana get along well. they are both victims & they support each other in this strange way.