I’m in an open relationship with my live in girlfriend (whom I love very much). Everything was working out really well, I even have a trip planned out of the city to give her some alone time with a boy she’s invited over.
The problem is I’m leaving on a saturday, and having a small get together with friends on a friday. And she says she can’t wait for Saturday to see this guy, so she’s invited him to my party. I’m not happy about it but I’m afraid that if I mention it again she’ll think I don’t want this relationship at all and leave me.
Is there anyway I can convey my unhappiness about the situation without sounding like a baby?
LemonVibe 
[I think it sounds like she’s holding you hostage with this whole “open relationship” thing. It bothers me a lot (and it should bother YOU) that you’re afraid to bring something up because your girlfriend will leave you. That doesn’t speak to your confidence in the relationship or the level of open communication you currently have.
So, this may be going a bit beyond the boundaries of your question, but how comfortable are you with the openness of the relationship? How long is this trip going to be? If you’re leaving specifically to allow her time with this other guy, I do not think it is too much to ask that she wait until you’re out of town to actually see him. And how do you do that without sounding like a baby? You just say it. Communicate. If you actually are ok with this whole situation, then I imagine you would say something like “hey, you know I’m ok with you being with other guys, but I’ve discovered I don’t want to actually see you with them. Also, since I’m going away, I would really like it if you could spend the last night with me.” As the primary (presumably) partner, you deserve to be able to state your needs and desires and have them listened to respectfully and handled in a way that is satisfactory to both of you.
Man, I really wish I could be as up front and honest in my own relationships as I advise other people to be 😉
[The most important thing I read about open relationships, time and time again, is that they have 3 main traits: trust, honest communication, and ground rules. Every person I’ve read about who talks about their open relationship stresses that ground rules are the key to the success of these relationships. You need to stop being afraid of talking to your girlfriend and start setting the boundaries.
Which brings me to an even bigger problem that I’ve noticed with your blurb: you’re afraid to come to your girlfriend and say, “Hey, I’m not comfortable with this.” This, in my opinion, is one of the biggest red flags in a relationship. If you can’t stand up for yourself or be honest when you have a problem with something, that is not a healthy relationship: it’s a relationship where you’re going to get walked all over all the time. And either that person is going to get frustrated being with someone who won’t stand up for themselves, or they’re going to enjoy making you do what they want with no regard to your feelings for the rest of their lives. A person who wants to be with someone who doesn’t ever assert themselves is not a nice person. So if you don’t stand up for yourself, either you’re eventually going to be alone, or you’re going to be in a relationship with someone who doesn’t care about you at all.
[I’d just like to echo several of the commenters on here, especially Lexington, BreckEffect, and resullins
[You had me at the contradiction in the first sentence.
If you love your girlfriend so much, why are you choosing to be in an open relationship with her? Is it that you’re trying to have your so-called cake and eat it, too, and realizing that perhaps you don’t have the appetite for such gluttony?
Or is your girlfriend the one calling the shots and choosing the open relationship?
Either way, it seems to me that the open relationship isn’t working, because you clearly have a problem with the openness of it. So, I think that’s the issue you need to address here.
[Tell her you have no problem with her seeing other guys as long as you don’t have to see them as well.
Unless, of course, you are into that.
[I’m exactly on par with what most of the other people said. If you’re uncomfortable with this, you have to tell her. This doesn’t make you a baby, or whiny, or anything. I will also echo the sentiment that you sound like you’re not really into this situation, and your girl is calling all the shots. She seems a little inconsiderate, a little arrogant, and a little apathetic about your concerns and feelings.
You need to talk to her. Not just about this, but about any and all the concerns you may have about this relationship and this situation… because it sounds like you have more than one. If she’s unresponsive to your hold-ups, she’s really not someone you need to be with in a relationship that takes so much compassion and work.
[Here’s what you do: let the guy come over, then approach him and tell him that your girlfriend was hoping for a little three way action with her two guys, and is he interested? That might put a stop to this lack of respect for your boundaries. 😉
[The key to an open, or any, relationship is communication. When you feel like you can’t communicate your true feelings, in this case that she’s prioritizing this other guy over you, then your relationship is struggling.
If you have time, talk to her now, before the party. Afterwards you’ll have those negative feelings and all she’ll be able to express is remorse. As a partner, if she “punishes” you for expressing your reservation, then she’s not committing to protecting your emotional/mental needs in the relationship. Good luck!