As long as you keep it in your imagination and don't actually try to humiliate anyone against their will, there's really nothing at all wrong with your fetish/fantasy.

There's been research done on the fantasies people have during sex and many many people fantasize things they would be totally embarrassed to talk about. In fact, it was discovered that women who could not orgasm easily or at all were often helped in reaching climax by learning how to fantasize unusual, non-conformist sex thoughts that had nothing to do with their lover during intimate moments.

Men are sometimes taught to think about unsexy things like baseball stats or cleaning the valves on their car in order to delay ejaculation if they tend to be too quick on the draw.

Each of us has the right to keep the contents of our private thoughts during intimacy completely private. You're not harming your boyfriend by using your own mind to arouse yourself and to reach orgasm. It allows both of you to experience the closeness of intimacy.

As long as you allow your boyfriend the privacy of his own thoughts during intimacy and you're not humilating anyone against their will in real life, there's nothing wrong with your tighty whitey fantasies.

The only problem I can see is that you didn't know that sex therapists actually teach both women and men how to use their own minds to improve their sex lives. You figured that out for yourself. You're all right.