If he's there all the time, and uses electric, water, and causes increased bills, that would annoy the crap out of me. I don't see it as jealousy, just a guy who is living off his girlfriend, and doesn't know when his welcome has worn out.

Have you talked to your roommate?? I'd broach the subject tactfully, saying that him being there a large percentage of time (and give specifics) is encroaching on your own privacy and that as he's not the roommate, he needs to limit his visitations, or they need to find somewhere else to visit. If she gets defensive, explain that you two are the renters, not he, and that if he is going to be there that often, he needs to assume some of the responsibility for the expenses he causes.

I would not be happy in your situation. I like my privacy. My home is my sanctuary, and if I can't be relaxed when I'm at home because someone else, who doesn't live there is always there, I'd be annoyed. If they insist on keeping the arrangement of him staying there 24/7, then I would find a way to have him put on the lease, and for you to be taken off and find another place to live. Or have them move out together and have you find another roommate (gosh, I really don't miss those roommate days!!!).

Wish you luck!!!