I don't know if ultimatum/veto is the way to go. I am in general pretty anti-veto, but I also think you will have to make decisions that are right for you.
I just wonder if it is going to bring what you seek. In the best case scenario, you issue an ultimatum and he chooses you. What does that prove... That you were, indeed, his priority? That you were more important to him than this girl? Is that what you want?
I think for me the ultimate goal would be to know whether he can be trusted and not so much to know I am more important than the other person. I don't think veto can give evidence about his trustworthiness. For that, you need to talk to him and listen to him. He needs to take responsibility, he needs to explain why he has made the decisions he has made, he has to have the willingness to act differently in the future. And then he has to show you that by acting differently.
In the end, I don't think the worst thing for you about veto is the possibility of him choosing her (because that may happen anyway if it's what he wants) or the consequences to her (which may be bad but if he dumps her he wasn't really a keeper from her point of view, anyway...). I think the worst thing is that the best possible outcome is the illusion of security. If, instead, you communicate with him and if both of you want to continue, you may have a longer period of more tangible insecurity, but the best possible outcome is actual security.
Does that make sense? Obviously, nothing is even 100% certain, but in the latter case, you can have some evidence that he has leart from his mistakes. In the former, you remove the symptom (her) and hope that destroys the problem (his actions).
Living with my partner Mya
and metamour Hank. Seeing Lily.