<human>: Can you succinctly summarize the text in 1-3 sentences?

My girlfriend and I have been together for more than 4 years and have never had issues having sex or being open to new ideas in the bedroom. She recently has gotten off her birth control and tells me that she is extremely horny as a result. Last night she brought up a conversation asking if there is anything I would be interested it trying in the bedroom that we had not done. I didn't really come up with anything, we talked about anal but decided it was not something we were interested in. 

Then she brings up having a foursome. We had talked about threesomes in passing before, not really being serious. It was decided that a threesome couldn't happen because neither of us are bi-sexual and would not be comfortable performing sexual acts with the same gender. The threesome conversation ended there and I guess it wasn't really made clear that I would not be okay with her being sexually active with another man, we more focused the bi-sexual nature of the act.

I think her rationale for the foursome idea was that we would each have a member of the opposite sex to give attention to. Upon her mentioning this idea I became visibly upset and I asked her if she wanted to have sex with another man. She said she thought it would be an interesting and fun experience for us but that she understood that I was not comfortable with her being with another man and dropped it.

Although she dropped it and said it was okay and that we didn't have to go through with it, the thought that she was interested and okay with having sex with another man is killing me. I talked to her about this and she said she understands and that she would never do anything to hurt me and it was just an idea that didn't mean anything. But it still bothers me that she wanted to be with another man. Am I just being a big baby and should let this go? I am not sure how to feel about it. I feel like I have already talked to her about it enough and that she understands my position, but it still hurts me and at this point she thinks I am just moping about it.
<bot>: GF wanted a foursome, I am not comfortable with her having sex with another man. Not sure how to feel or what to do.