he may have already been seeing someone, or dealing with an ex.. some men just lose interest after sex because that is all they are looking for. This has happened to me and a month or so later I seen the guy at Ihop with what looked like his GF
This is true my bf of several months started to complain about me wanting an appetizer at dinner, which I knew for a fact that if he was still trying to impress me he would not make a deal about it.