With the greatest respect and honestly no offence intended - why are you being such a little bitch, dude? Firstly you telling her was a mistake, there's no way to say that without coming across as possessive, needy, jealous & whiny, because that's exactly what it is. IME there's no faster way to lose a girl's interest than acting like a bitch, and I don't blame them. They want a real man, a secure emotional rock, someone who they can feel safe with because he's confident & self-assured. You start talking to her about your feelings being hurt by her wanting to see other guys destroys all that, and you turn into a needy little boy.
Not only is that needy & whiny in itself, but the implications of what you're saying are also gonna turn her off. It would be different if this was an ex she was hanging out with, or the guy had a reputation as a player or whatever, but these are just normal co-workers by your own admission. Do you know how insecure that makes you sound? The implication is that if you don't keep her under lock & key she will suddenly fall for somebody else. As if she's not into you for your positive characteristics, but merely because you happen to be available and in front of her, and the minute that she hangs out with other guys she's going to realize that you suck and that you'd tricked her into dating you, and she will be sucking her co-worker's dick while the other one pounds her ass within minutes. Do you see how this is all implied in your jealous whiny implicit accusations? If you were confident in yourself & secure in the knowledge of her attraction to you, you would have no reason to be jealous. You'd be happy that those guys had warmed her up for you, doing the talking and the joking so she comes back and has sex with you. Instead, you're upset and jealous, and it must be an enormous turn-off for her.
I mean, this would be different even if she was going to hang out with one of them, that could then be easily construed as a kind of date situation, and your response would be a little (not totally, by all means) more justified, and the same if one of them bought their date/girlfriend/partner and the other guy was single. But holy shit man, she's going to hang out with two co-workers who just happen to be guys, it's a really innocuous situation and the fact that its upset you shows your insecurity and paranoia. If you keep acting clingy & possessive you're gonna lose this girl, mark my words, her telling you that she will have more guy friends in the future was her trying to explain that you should BE A MAN, and accept it. You clearly care about this girl, else none of this would have bothered you in the first place, and I think the best way you can hold onto her is by working on yourself and gaining confidence. If you become happier with yourself and are self-assured that you're a catch, you're less likely to get all jealous and weird and insecure when your girlfriend wants to hang out with other people. This is a much more constructive way of keeping your girlfriend interested in you - improve yourself, make yourself happier and she'll want to be around you & stay drawn to you. Getting upset & jealous that she's hanging out with others and getting possessive and trying to keep her boxed in with you will just repel her and have the opposite effect of what you want.
So you fucked up majorly here, but I don't think it can't be salvaged. As soon as possible you should apologize (once, not profusely) and tell her when you thought about it afterwards you realized that it totally wasn't a big deal at all, and that you don't know what got into you, that your negative emotions weren't really anything to do with what she was doing and that you just had a bad day. That you realize what you were saying was ridiculous and that that isn't how you feel. That should hopefully nullify the damage that you've already done. Finally, do you have any female friends? If you don't, you should, so go and make some. Don't make it obvious by doing this while she's out with these guys or in the same week or whatever,but arrange to meet up with them to hang out at some point. Don't make a big thing out of telling your girlfriend, just casually drop it into conversation at some point. She will either be totally cool with it or will have to pretend like she is anyway, and you can go and practice being flirtatious and keep a couple of backup options around in case anything happens with your partner, and you can just go and have a good time anyway. It sounds like your life revolves around hers, this way you can have your own life so you won't be so concerned with hers, and if you have a good time of your own with some female friends, and you entertain them and they enjoy your company, you'll again feel more secure & happy in yourself and your girl will be happier with you too.
tl;dr ur bein a bitch stop that
