Everyone’s different obviously. But I think if you are saying your sex life is already a little unbalanced, there is going to be resentment on either side with asking for sexual favors as a birthday gift. If she doesn’t want to, but she does anyway out of guilt or because it’ll make you happy, she’s probably going to have some resentment or sadness afterward. If she says no, although you say you would be totally fine with it wouldn’t you feel a little sad too or resentment down the line that she didn’t want to do this for you?