I would consider using a veto in two very specific cases;
1) adding another partner would seriously take up already thinly-divided time. Say, my partner would consider taking on a fifth partner. I say four is pretty much the upper limit I can see one person engaging with at the same time and still having enough time for work, hobbies, other duties etc.
2) continuing a relationship with one partner is seriously threatening the wellbeing of everyone in the constellation. I'm thinking drug addictions, trouble with the law, acute mental health breakdowns, compulsive lying, cheating, violent behavior, something of that magnitude.
Using a veto in any other case, because of insecurity, jealousy etc. I think undermines the foundation of the relationship and might lead to resentment and boundary-breaking. Haven't thought the implications of absolute vs. negotiable vetoes. Are you thinking of something along the lines of a person A being off-limits for now, but maybe not so in the future?
Me: bi female in my twenties