If she wants it that bad your relationship probably doesn't have any future anyway so tell her that if she really wants a threesome it'll have to be with her next boyfriend, not you. OR, insist that if there's gonna be a threesome it'll have to be another girl. See how she takes that. Either she'll feel the same way you're feeling now, or you'll be with two girls at once.

This is a big issue and not one to be jumped into. Although I'm all for trying things at least once, when other people are involved there needs to be extra caution. Her light view of 'it might be fun!' seems to be more in fantasy (the appeal of that fantasy). Tell her she needs to realize/take into account other things that can happen in reality if you guys do this. You already feel that you don't want/don't like the idea of other guys being with her so if it happens there will probably be a lot of jealousy/insecurity/trust/possesiveness issues coming up strongly. She needs to realize this. In the cold light of day after the night before, things will be different as opposed to her fantasy of it. You need to tell her this. Also, like the last person said, suggest a girl instead...ask her would she feel the same you do with not wanting to share her guy with another girl. There are other smaller ways to indulge it, if you're ok with dirty talking about it when you're having sex with her, etc. Bottom line, I think any couple or part of a couple who is interested in inviting another person in has to sit down and have a LONG open talk about it beforehand. It's not just like say a foot fetish that you want her to indulge. In your talk, you need to bring up any possible thoughts/feeling you have on the issue and if you decide to go through with it you have to establish whatever ground rules you have (better to beforehand than to be in the situation at hand, have something happen that's over the line, and then blowing up at it after it's happened because no one knew. Make sure no one crosses any lines). It all depends on the couple. I know some that have had great fun with it and it's enhanced their sex lives but I also know of some that is has destroyed it or did a whole lot of damage. It should be consensual on bith sides. In my case, we occasionally invite girls in and I have no problem with my man being with other women, in fact I love watching him with other girls and joining in as well; on the other side, we don't ever invite other men because he doesn't want other men being with me BUT I, myself, do not want to be another man at all so it's not unfair as some people may suggest. See, it's consensual. Tread carefully and good luck :)

If you are not into it, and she is... and it keeps coming up... it might be time to move on. There are lots of women out there, just as smart and attractive, that are into what you are into. This will not go away.

Obviously, having a threesome is one of her fantasies. There is nothing wrong with that. Did she say she wanted it to be MFM? You may be getting all worried over nothing, maybe she wants her girl in the mix. If you have had a talk about how you are not comfortable with having a threesome, be it MMF or FFM, there is nothing more you can do. She should really be respecting your wants, and wishes. It's not like your the first guy to not want another man to fondel his woman. If she contunies to hound you for this, maybe she is not the right girl for you. Good luck!

If you dont want to have threesome and you have talked to her about it and she continue to want one then it time to move on. I know it hard but every relationship have boundaries and if that one of your boundaries then tell her so long and stick to your guns.

Tell her that you are bothered by the idea nd it makes you really uncomfortable. You could say, "More than two people in bed is too many." I know you've already talked to her, but saying it again will help reinforce your points. You could also try to work with her. Creatively using a dildo in the bedroom could help to create the illusion that there's more than one guy in bed with her. You could also ask what her other fantasies are, and if they're a little more tame, try to satisfy them. Then she knows you're not boring in bed.

All good answers and I may be a bit late on this, but if she wants to try it that much, and you don't want to find out what she wants MMF or FMF. If it is MFM, then it is your choice. I can understand you not wanting another man to be inside your woman. I felt the same way when my GF and I talkied about it. The thought of seeing your woman get pleasure from another man, or that maybe after it's all over a spark lights up between them, or your left wondering if she enjoys him more then she enjoys you. Sit and talk to your better half before and tell her how you feel, and your worried something might happen with her and this other guy. Talk for days, you need too, and then think of something you really want with your sex life. Is there a girl on the block you saw and would like to fuck. Well tell your woman. I will do this for you if you do something for me. Tell her about the girl down the road that you always wanted to fuck, and that you will have this MFM 3 sum if she watches you fuck this other girl or has a 3 sum with this other girl.