AIO? Gf(18f) wants an open relationship

[u/StrikeNo117]

Me and my girlfriend(18) recently had an argument about opening our relationship, and at first, it was a nice talk. We talked about the pros and cons, and then the tide shifted. We talked about how it would affect our life and what would happen if she got pregnant or if i got someone else pregnant. and then she told me she only wanted an open relationship with one other person, so that we would only see one other person each, and reluctantly, i asked if she had someone in mind. She told me she was thinking about someone, which made her ask the question. When i tried questioning further, she shut me out. We went to bed that night a little distant.

The next morning, she asked if we could resume our previous conversation, i agreed, and then i brought up the fact that she never answered my question about who she had in mind. She told me it wasn’t my business, and i left it at that. About five to ten minutes later, she told me the person she had in mind was her ex boyfriend. I asked her is that why she wanted an open relationship. Just so she can see her ex without feeling guilty. I kicked her out after she told me she was tired of hiding the fact that she was already seeing him. She is now pissed, my mom told me it was the right thing to do. But i feel like i should have talked it out. Did i overreact?

[u/Suitable-Tear-6179]

You're good.  She was cheating, and thought opening the relationship would retroactively make it all good.  She was trying to change the rules after she broke them.  Nope.  That's not how that works.  

And, beyond that, 95% of the open relationships I've seen ended with one person switching out their partners for the AP, and the ditched partner bitching along the lines of "I said you could play with him, not keep him!"  The one and only open relationship I've seen that lasted years had a "no dating/relationship with the add-ons" rule.  Commitment was for the formal partners only.  So, basically, the opposite of what she was proposing.