The way you make it sound, your relationship breaks down into 1. She's nuts. 2. She's amazing in bed. Nothing about how well you get along, how much you care for each other, anything like that. Sounds like you're freaked out, maybe even a little scared. You don't sound like you're particularly happy in the relationship, and that the only thing you're getting out of it is sex.
Well, are you in it for long-term? Where do you want the relationship to go? Where does she want it to go? Are you little more than buddies who also screw, or are you a genuine pair? The way you make it sound, you want to screw her, then get out and have nothing to do with her. If she's fine with that (which I doubt, since few people are) then more power to you. Otherwise, you're using her. And if you are using her, then apparently her paranoia is at least partially justified- because here you are sneaking behind her back and talking about her while you have no interest in anything about her but her body.
People have sex all the time, bless 'em. But if you think you are in anything approaching an emotional relationship, it behooves you to treat your girlfriend with the same level of respect and dignity that you would wish. And part of that is honesty with both her and yourself. This does not mean being a nasty idiot. If you decide to break it off, you do it in as gentle a manner as possible. But unless you are in a clearly defined purely physical relationship, then sex as the only reason to stay together is a terrible basis for a relationship.