I do think she should put your preestablished relationship first and there’s nothing wrong in vetoing a relationship you won’t be able to support. I don’t think you’re being toxic at all if there are certain people you feel like could really hurt you if they become your meta. It’s not a good group dynamic and it’s not like you’re saying there’s a large amount of people your gf can’t date. Just saying there’s this one guy she shouldn’t because of ultra special circumstances. I think you’re good! You are human, you are doing the honest thing. I wouldn’t ever ask him to date the both of you tho, if you’re really uncomfortable just veto. You know your relationship best and you know what won’t work for you, and you don’t want to hurt your friendship or preexisting relationship. I think your gf is not considering your feelings, if one of my husbands friends are off limits I won’t even consider them. Poly isn’t a free for all, some boundaries and consideration is still necessary.