<human>: Obvious throwaway...

Anyway, here's the deal: My gf (22) brought up the idea of having an open relationships. She wants to "explore sex" i.e having 3somes, and sex with other men and women. I am free to do the same, obviously, if this were to become a thing. I'm kind of blown away by this because she was a lot more conservative when we met. She was actually a virgin and we have been together 3.5 years in a  long distance relationship (different schools, 3 hours apart). 

I understand her curiosity, often I feel the same way wondering what it would be like to have sex with other women. I'm not interested in exploring gay sex, but I'm not against her doing the same (if this were to become a thing). However I feel like if this were to be explored it would be the end of our relationship. She seems to be fine with the idea of me having sex with other women, and even considers it a turn on. But when I think of her having sex with other men I just don't think I would like it. I can accept the idea of a 3some much easier than casual sex with others solo, because at least then we would be doing it together. 

I should also note that our sex is good for both of us. We are both able to orgasm so this isn't because I'm unable to please her in that sense, but as long as she has only been with me there will always be that curiosity in her head. She also says that if I decided against it she would forget about it because she values our relationship more than her sexual curiosity.  

I don't know if I need to get over this apparent alpha-male attitude of other men having sex with my gf, or if my feelings are natural... Looking for advice from anyone, but if you've been in or are in a successful open relationship I'd love to hear from you

Can you summarize the given text in a few sentences?
<bot>: Gf proposed open relationship. Not sure if I'm comfortable with it, and not sure what guidelines to follow to make it work if we did decide to try it.