A.J.
Ok, mama. Yup, of COURSE they're having sex. Along with your other descriptions, sounds like you have a normal and responsible ADULT on your hands. Bravo for the job, the responsible nature, and the nice personality. Good guy! Yes, 18 is adult.
ALTHOUGH, adulthood comes not only with the 18th birthday, it comes with supporting himself. As long as he lives in your house, he needs to obey your rules, curfews, requests. That's why we all leave home isn't it? Time for him to go! He can be as tired and overextended and sexed up as he likes- yahoo! being young is sweet- but most of that fatigue needs to come form the 2-3 crappy jobs he's working to pay his rent and bills. I did it, my husband did it, the sooner he gets out there to do it, the better off he'll be in a couple of years as he climbs the job ladder slowly. With his girlfriend as a nice perk to his freedom. It's not your job anymore to teach right and wrong-you've done a great job. But he does need to follow house rules until he's on his own.
The longer he lives under your roof, eating your food, falling short of your expectations on family duties due to fatigue, and spending all his spare energy and time on his girlfriend, the more strain you're going to feel. He's an adult. He's doing all the growing away he's supposed to be. Get your plan in gear for his gentle departure, and don't be afraid to assert yourself while he's still home. This will help him leave. My husband always remembers the day his mom told him he could pay rent at home with all her rules or somewhere else with his freedom. He was gone the following month! :) Good luck! I'm dreading that day, and my son isn't even 2 yet!