How are things with Alex? He/she (?) gets only a brief mention, which centered on sexual incompatibility. I get the impression you're not happy being with Alex. I could be way off the mark, though. Who knows?
Playing with Sam seems like playing with fire. This is exactly why I don't do the fuckbudy, casual sex, thing. If the sex is good then I'm emotionally involved. If I'm not emotionally involved, the sex is no good (for me!). If it is a relationship I want, with sex included, then it's best to start out as good friends (for me!).
Maybe you can transition from sex buddies to love partners. Maybe not. But what is the deal with Alex? Where does Alex fit in to this picture?