I think all of this talk about him needing therapy and his desires being "unhealthy" is all a bunch of horse shit, and you need to quit suggesting it to him. All this does is make him feel like he's a freak and repress his desires further. Repression is unhealthy. I'm NOT suggesting that we all indulge our deepest darkest fantasies, because there's a lot of fucked up people out there who would try and justify outright rape as "indulging their fantasies." I've got the cuckold fantasy too and I'm what society would label as more towards the "normal" end of the spectrum....little do they know. And I've never been raped nor mistreated as a child (sorry to hear about that BTW, F'in child molesters should all be strung up and fed their own nuts).
In the OP's case, his fantasy deals with himself and his GF's treatment of him. The only one who stands to be harmed in this case is himself.....but here's the thing....HE'S ASKING FOR IT. He wants that humiliation - he craves it. And he may never be fully satisfied until he gets it.
AS long as it's consensual...I don't see a problem in indulging this fantasy every once in a while. Every time he's sexually intimate with his GF it doesn't have to be this fantasy, but every once in a while is reasonable.
To the OP: your biggest challenge is gonna be to get your girl to actually go forward with this...AND MEAN IT! Nothing is worse than when someone tries to placate their partner by doing something, but isnt really into it, and so only half-asses it, and apologizes the entire time. I've been there, done that...it sucks. Either go all in or GTFO.
Have you sat your GF down and sincerely explained to her how much it means to you for this fantasy to happen? If you have brought it up, how has she reacted?
Total honesty without apology is always the best option. We are who are are. We all like different things. Doesn't mean we have to fit society's label of "freak," or "weirdo." Fuck society's labels.