If everyone is mature and well balanced I don't think there is a need for veto at all. If an unhealthy situation is arising I think it will be recognized by logical people. In the case of a person becoming involved with people in an established relationship, I think they will be strong enough to step back if they sense their presence is disruptive and/or destructive provided they are self sifficient and self confident.
The danger lies in the case where one or more of the participants is not self sufficient or self confident and will remain in an unhealthy situation because they see no other way to have some one.
We're all adults I presume; I expect we will act accordingly and make healthy choices even if they are the ones that hurt us temporarily.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over