
D.B.
It would help to know what you're most worried about. You focus on the girlfriend and her looks, so are you worried about sex? He's 18 so you probably have done a whole lot of good talking with him up to this point, right? He's had health classes and good family talks over the years about responsible behavior, right? He knows about protection against STIs and about contraception and about not taking photos and especially about taking "no" for an answer, right? There's nothing he will do after, say, midnight, that he can't do at 3 PM. So you can't be using a curfew to control behavior at all.
I think curfews have more to do with what else is on everyone's schedules, if he has school or work the next day, or if coming home late and setting off the family dog's barking will wake everyone else up. And I'd have a different curfew on weeknights than on weekends.
There's not much info in your post so we don't know what you've done up to this point. If you want to edit your post to add more info, that would be great.
But the fact that he has a girlfriend or that she's very pretty has zero effect on his behavior. So in my mind, that should have zero impact on your imposition of a curfew. There are temptations all over the place for kids at 14 and 16 too, so there's nothing magic about 18.
I'd be more vigilant about things like drinking and drugs and driving while exhausted. He's an adult, so your parenting really should be pretty far along by this time.