I'll start off by saying this is going to be a tad long...I'll try not to ramble too much though.

Backstory: My partner of almost four years (very serious relationship) lives a couple hours away from me--finally has his own apartment. It's his last year of college (five-year plan), and I graduated in May and am back living with my mom until I find a job, hopefully where he is.

Anyway, a bunch of our friends from school were over visiting this weekend (alas, I couldn't get down there). But then he told me one of them would be spending the night tonight when everyone else went back (she is the only one there this weekend who's no longer a student--she graduated when I did and now actually lives in my hometown). He said she can't spend the night with one of our friends back at school, which is why she's at his place tonight and not there, but we have our other friends at school I'd think she could stay with...

I'm a tad uncomfortable about this. I mean, it's not like I think they're going to do anything, but still. I'd be fine with it if I were there, or if one of our other friends was there, but....

It makes me feel weird when they hit on each other a lot, which they do. I'm not entirely sure if they sort of have a crush on each other--I am not sure if she finds him at all attractive, but I suspect that he either has a tiny crush on her or that he just plain likes the attention (she's the flirty sort, so she hits on everyone, but he really seems to eat it up). It's a little weird to me, to see my partner sort of hitting on someone else, whether it's poking each other and giggling madly, or when they'll give each other a "meaningful look" sometimes, the sort of way you wouldn't expect someone to do except with the person they're with, you know? And she's one of those girls that is attractive to guys, so it's not like it's entirely surprising. Even though I know nothing's up, and he doesn't ignore me when she's around or anything, it freaks me out a bit. But, like I said, I'm willing to be fine with it because I know it means nothing and just chalk it up to my jealousy/insecurity.

However, this spending the night thing. I know nothing's going to happen--even if they get really drunk or something, I'm pretty sure--but it's still weird. I mean, he's pretty strange when drunk, but I think he'd be with it enough even if a girl he found attractive made a pass, and while I know she probably gets lonely since she and her boyfriend broke up a few months ago, and I think she's sort of the type that might find it amusing to steal someone else's boyfriend, I'm sure she's NOT the type to do it to a friend ( I can't get the thought of "what if something DOES happen???" out of the back of my head, but that's because I'm weird and have always been irrationally afraid of infidelity). So before anyone suggests this is a trust issue: it's not.

I guess a lot of what bothers me is how it will LOOK. I mean, the residents in his building are probably used to seeing me when I visit, the apartment owner and manager know who I am and that he and I are together...but really, if he has another girl spending the night, what is it going to look like? I don't want weird looks next time I go back there because people are thinking "ooh, poor thing, I bet she has no idea he's got another girl..."

I know people have mistaken them for being together before--they went to a ren fest once that I nor any of our other friends could go to, and somebody they talked to there assumed they were together, according to my partner. (I didn't want to ask him why the person thought that--I preferred to think it was one of those things where this person always assumed that two people of the opposite sex who happened to be somewhere together were in a relationship, but considering the way they act sometimes I imagine it would be an easy mistake to make. This too bothered me a bit, but again, I'm insecure so I pushed it aside).

Am I stupid to feel like this? I mean, is it prudish to worry about how something like this will LOOK to other people and what they'll think of it? Maybe it's just me--I mean, I've always sorta been raised that you don't exactly spend the night one-on-one with random guys you're not in a relationship with, you know, even if they're friends--it's not something I can imagine me ever doing (I know my mom would be horrified if I told her this was happening--not sure how his parents might feel about it if they knew). And it probably wouldn't bother me so much if it was one of our other friends, rather than this one, because of the way they act around each other.

So I don't know if I'm overly jealous, or overly prudish, or what--I know I'm always too afraid of being cheated on, which is why I'm not sure if I'm just out of line, or if it is really not a cool thing to do. I have the urge to say "uh, can't she stay with one of our friends back at school? Because this doesn't really seem cool to me," but I also don't want to be the overjealous, overcontrolling b*tch of a girlfriend who won't let her partner see any of his friends, and I don't want to grudge my friend a place to stay when she's down there, but....as I said, I'm not sure why she couldn't stay with one of our other friends just fine (I also don't want this becoming a common thing, for her to stay with him when she's visiting).

Would you let your significant other have someone of the opposite sex (or same sex, if you're not straight) overnight, just the two of them? Is that quite, um, proper, for a guy who's practically married but for the legal part, even if nothing untoward happens? Should I just STFU already and suck it up and stop being such a jealous controlling b*tch?

TIA for all advice...