I don't think you know what the term "jealous" actually means. You see, when you're jealous about something it's because you covet or envy a person (or thing). There's a rivalry of sorts regarding another's success. You, for lack of a better term, own your man's shlong. You have it, essentially, to do with as you wish - whether that be to ogle it, kiss it or play gin rummy with.
 
Women -- who are not you -- perhaps can speak of jealousy. "I wish my man had that type of trunk-junk." "Get a load of the meat on that butcher." But you, as the girlfriend to the shlong, have no reason to feel jealousy. Perhaps you can explain what the emotion you're feeling really is. Is it pride? Protectiveness? Insecurity?  He's not brazenly flaunting his prowess, correct? He's not only wearing tighty-whities at the office, or measuring out carpet with it, right?
 
Let's put it this way:
 
If you wanted to be a football player and your brother was a star football player, how would that make you feel? Jealous? Fair enough, but if you don't have a desire to be a great athlete, and you hear the crowd cheering for your brother's skills on the field, how would that make you feel? Not jealous, right?

In other-words, your man's got crotch-game (c). Be proud of him. So long as he's only using it to score with you I say touch-down.