It's really not anything to be ashamed of. Other people like funny stuff too, they just don't tell you. Even if you like something more mainstream, like doggy style or whatever, and that gets you off, it's still pretty much a fetish isn't it?
As for thinking about feet, and it making you unfaithful, don't be ridiculous. It's not even an issue if you think about someone elses feet ffs, it's in your head.
Actually masturbating ONTO someone else (other than your gfs) feet would be cheating yes.
Now honestly, in your position I wouldn't tell her. You don't need to and I think that sort of thing is only ok to explore in a loving relationship thats going to be around for a while. If you two are relatively new to each other, that's probably going to seem a bit scary. I don't personally have anything like that though, so I can't be sure.