Depends on the guy, there are several possible answers for his behavior, from innocent to guilty as Hell. And another factor is how secure you feel with the relationship (assuming you have a personal interest in making this your first post on the very day you've joined RJ).
I'm partnered and yet active on this site, listing myself as having a monogamous relationship. I tell my partner all the goings on here. Even bring him over to the screen to read stuff, though he's not interested in being active here himself. When I meet guys from here for drinks or dinner I always ask him to come along, which he does.
So he has no concerns about my being on this site (which is not principally a dating or hook-up site, though it can certainly be used for that purpose). He might be less comfortable if he saw me describe myself as single, and worse, saying I'm here to meet guys. And he happens to be on Silver Daddies himself, where he has friends from before he met me. And so I told him to stay there, enjoy his online friends, don't drop them because of me, I don't feel threatened.
The best spin I could put on that kind of listing you indicate would be someone who likes to tease, to still play the game. Who needs lots of flattery & attention, which he might not get if listed as already committed.
You might ask yourself if he has that kind of ego, and if you're failing to stroke it enough (I meant stroke his ego, but maybe the other meaning is applicable, too). BTW, how do you know what sites he uses, and how he lists himself? Snooping would suggest the relationship is already in doubt. Or is this yourself we're describing, you the one whose actions are in question, wondering how he'll react to it? The same comments apply to him with you.