You are correct in asking why someone would be a member on a dating site when they are dating someone, but that would imply some kind of exclusivity agreement has been made between you. If not, then this is not an outrageous thing as you have not agreed to be commited to each other exclusively yet. If so, then I ask what the other poster said, he could ask you why you're on there too. Why do YOU have a membership there still? Again, he could be checking on you too. You never know. Try to see both sides of the situation and communicate with him how you feel about it as the other poster has said.

Also. As it has also been asked, how is the rest of the relationship, well I'm going to go out on a limb here and say that if you're checking on your partner to see if he's frequenting a dating site still, this speaks volumes to where you are, and where the relationship is. Be careful about becoming somewhat of a stalker like gf partner that checks up on her their partner to find things questionable without reason. That only leads to suspicion and mistrust. If you're that insecure about where you stand with this guy, this is yet another underscoring factor that communication is needed and you should know up front where you stand with this guy before moving on.