J.B.
My oldest son is 18 and still lives at home while going to college and working. His girlfriend, who also lives with her parents while going to school at a local college (excellent college with housing but her house is much nicer than a dorm so why live there), is here all the time. I don't mind it. The way I see it, he's saving me a ton of money by living at home and attending an inexpensive community college for a couple of years before heading to a larger school, so I'm fine giving him some of the freedom and leeway he would have if he lived in a dorm or an apartment. If he were living away (like my SD is to the tune of tens of thousands of dollars a year), he would have friends over, they would make food, hang out, watch movies, study, etc. It was a little jarring at first to sometimes have his girlfriend come bounding in with breakfast for the two of them at 7 am on a weekday, and I often go to bed while they're still here (both adhere to a midnight deadline to be in their respective houses), but overall, I don't mind. They're young adults living their lives. If they were away at school, I wouldn't know any of this. I don't feel the need to curb his freedom too much just because he chose a more practical road to adulthood that is much easier on my wallet. My house is by no means a dorm or frathouse - there is no drinking or substance use here, she doesn't stay over, they don't spend time in his bedroom, etc. If the worst thing two college-age kids are doing is cooking and hanging out, you're doing just fine.
FWIW, my son's girlfriend knows that she doesn't get the "company ready" version of me or my house. I treat her like one of the kids and she sees me without makeup, in my ponytail and sweats. Maybe in a few more months you'll get there with your daughter's boyfriend. I'm not saying that you're wrong to feel how you do, just wanted to offer another perspective. I know that my son and his girlfriend appreciate that they are always welcome to be here.