Professional assure us that masturbation is just about quality time with yourself, and the relationship status is immaterial. It's normal and healthy, and reflects nothing on you; I think Battleship could have a point that perhaps he was horny enough to want an orgasm, but not horny enough to make it into a whole quickie. Also? It's often good for guys to come once or twice before having sex, as they can last a lot longer that way. Maybe he just wanted to have more stamina later?
Also, if it does bug you? You can talk to him about it if you can be nonaccusatory, and maybe come up with alternatives if you want to be involved. Maybe if he doesn't feel like sex but does feel like orgasm, you can help? Nibble on his ear while he masturbates? Or perhaps you could just play vouyer, if he wouldn't mind showing off? Maybe you two could perform for each other? Throwing out ideas. But as Anansi said, don't forbid, because that's a) not fair, and b) far too controlling.