I recently started seeing someone who’s poly, which is new for me but not a problem. But he’s been flirting with a close friend of mine lately who’s the the one person I’d be uncomfortable with him dating. I told the friend early on that I wouldn’t want anything to happen there and they said they’d respect that, but have been flirting back anyway, which I feel a little betrayed by. I haven’t told my partner about this but he’s said he doesn’t believe in vetoing things. How should I handle this?
You say you haven’t told your partner about this - you need to do that!!! You can’t make huge assumptions about how he’d respond based on what he’s said in the abstract.
There is a difference between having a “veto” rule vs. responding with grace and sensitivity to your partner saying “this makes me uncomfortable.” You aren’t telling him that you forbid him from dating this person, just that you’d strongly prefer that he not do that.
Let him know how you feel, that you know he doesn’t believe in vetoing, but that this specific situation really bothers you and you want to find a solution. Then, if he decides to go ahead and date your friend anyway, you decide whether you want to stay in the relationship. But first, you need to talk this out with him.