To me, a fetish is something you "NEED" to get off. Just because you enjoy someone sitting on your face, to me, doesn't mean it's a fetish, unless it is the ONLY way you can get off.
Personally I love eating a guys ass and I love when he sits on my face and blows me in a 69. I can do that forever, or until I get his ass horny enough to turn around and sit on my dick and penetrate himself with my woodie. I also like it when the bottom grinds down hard on my dick and smashes my balls trying to get every last bit of my dick in his poop chute!
I enjoy lots of crazy stuff that I'm sure some people would call a fetish or crazy, like jocks, wrestle singlets, spandex, lycra bike shorts, wearing wrestling masks, double fucking a FleshLight with a friend, toys, vibrating butt plugs, older/younger roll playing, etc. but I don't consider any of it a fetish as I don't need any of that stuff to get off, it just adds variety and it's fun to do different things.