Is it just sex, or is it possible that he likes me?

Been casual friends with this guy for 2 years. I have always had a crush on him and he knows it. We are both recently divorced and I was hoping I would get a shot, but he ended up with a girlfriend pretty quick. However we have started sleeping together, we flirt constantly and he invites me to stay the night at his house. The girlfriend doesn't live in town. I don't know how serious they are, but she has met all of our mutual friends and they "love" her. He fixes my car, my lawn mower, we send naughty text to each other all day. I guess my problem is that I'm heavy and she is a tiny little thing, and I may be paranoid, but I feel like he has the "show" girlfriend and the "no show" sex buddy. None of our friends know we're sleeping together, and we never discuss it in any public place. We are either at his house our texting. Is this just a total sex thing??? Should I stop waiting to see how long this girlfriend will last?