You are not being hypersensitive. It is totally inappropriate for him to be acquiring these "friendships." It doesn't matter that his intentions may be totally innocent, as I believe they probably are. The pattern he's engaging in will eventually lead to trouble. It's only a matter of time before some girl he "befriends" has more than a "friendly" interest in him. (I'm assuming he is an attractive guy, which is part of why you got interested in him.)

He may have no special interest in any of these girls now, but it's just a matter of time. Couples have their ups and downs, so the two of you will have yours. It's part of life. One day, when you and he are a little ticked off at each other over something, one of these girls will be ready and willing to show him some attention that will feel good to him. Suddenly there will be 3 people in the relationship. Not good.

Whether he has the wisdom, or not, to see what's wrong with this friend-finding behavior of his, you should go ahead and tell him that it can't continue, if he expects you to stick around. I doubt he'ld be okay with you doing something similar.

A certain amount of jealousy is normal and protective of a relationship. Go ahead and tell him - or admit to yourself - that, yes, you are jealous. It wouldn't say much for your relationship, if you weren't.