Part of real love is being able to say to your lover, "I trust you with control over who I sleep with, because I trust you to make your decision based not on your own insecurities but on a real consideration of my needs, wishes, and safety." If you do not have this level of trust in them, you need to pull back from polyamorous adventures and work on trust-building within the relationship.
I've always been against the veto since I don't agree with an outside party being able to end a relationship, but I thought this was an interesting point of view - the idea that you trust someone enough to make the right decision by you instead of the right one for them. I'm still not sure I'd feel entirely comfortable giving someone that power thought. Thoughts?