It doesn't seem like you've looked at this from your girlfriend's perspective at all. On your end you've done lots of research and formed an educated opinion on the subject over time.

From your girlfriend's perspective you came up to her and out of nowhere said you want to fuck other people. She immediately reacted negatively which is to be expected from a monogamous person. She asked if you were cheating on her, you said no. She asked if you wanted to cheat on her (which pretty clearly shows she's not okay with the idea) and you said "no, but actually yes with permission."

Then you said "wait, you can also do this thing that I want to do that you're currently expressing disinterest in!" She again reacted negatively, obviously, and asked if you'd dump her over this. And you said that you probably would.

Yeah I can't think of anyone who would react positively to this situation. If you had first begun having these thoughts and expressed them to her, while prefacing that these are new thoughts and you absolutely would never go behind her back for this and other ways to reassure her and address subsequent insecurity, it might have gone differently. Instead you ambushed her with a debate strategy.

Did you think about how your girlfriend feels about monogamy already, how she views sex and intimacy, if she has any insecurities this might trigger, how to explain this to someone unfamiliar with the subject, how you would direct the conversation if she wasn't open to it, or anything else about her that you would need to discuss when bringing this up?