Again, thank you all for your support. I have talked a lot with my girlfriend. I can share my fears and insecurities and it has been rather liberating. She is exploring (well, a bit more than just explore) getting involved with someone in her hometown (I was aware of this before hand, before getting serious with her). While I do know she has a few affairs of the hearts and even a one relationship, this particular one scares the daylights out of me. Part of it has to do because he is in the same town as her.
She has offered to take it slower (which I accepted, but I also told her I trusted her to make the right call) , but I cannot keep but feel threatened by him
We are still establishing boundaries, but I don't want either to hurt her by saying "No, you can't date him", even though for this specific case, part of me wished it was so >.<
I did ask her to have a talk with him, because he is also polyamorous, married and living with with his wife and his wife's boyfriend. I asked her to let him know that I am feeling very threatened and ask some advice on how to reassure me. I think it is important that if he is going to get involved with her, he also knows how I feel about this matter. She has agreed to have a talk with him, which comforts me a little
She is going to an event with him tonight and I expect to hear from her only tomorrow (she will likely spend the night at his place because there will be drinking involved and no one would want her to drink and drive).