"I Want You To Stop Dating Them" is an IG Post on Marjani Lane's page. This is the accessible read-only text.
The Veto
A veto is the established (or assumed) power a person wields to end their partner(s)'s relationships. These relationships can be romantic or s3*ual. Vetoes are considered unethical because someone else's relationship can be ended by an outside influence. They are typically used in reference to polyamorous relationships but can also be done in other forms of non-monogamy. Vetoes are essentially orders and ultimatums that can incorporate emotional manipulation.
"Stop dating / talking to - insert person(s)." "If you don't break up with them, I will break up with you."
"If you love me, you will leave them."
Position of Power
Typically the person(s) utilizing VETO are in a position of power in relation to the metamour and outside relationships. They may be a primary partner(s), spouse, anchor and/or someone that has some form of power and influence over their partner(s). The loss of ignoring the veto may be greater than just simply leaving the relationship - and unfortunately the affected person is expected to pick.
The Logic
The rationalization behind veto existing in a relationship varies. Some partners feel like their lovers are unable to make healthy decisions in their outside relationships and feel they need someone to do that for them. There may be a gender superiority complex where it is assumed one of the genders would know what's best for the relationship. Some people may argue it is for the protection of a partner going through an abusive dynamic. A veto may exist to validate the importance of primary partners as having the power. It can also be used a threat to maintain order in the relationship if someone violated boundaries.
The Issue
It is simply problematic to create a new relationship that is at the beck and call of another unrelated relationship. If a partner who is in a position of power is feeling uncomfortable, jealous, envious or simply does not like their metamour - they can veto the relationship. Someone's entire relationship is disposable at the will of someone outside of it. this can cause animosity and anxiety.
The Alternative
Establish boundaries and expectations with your partner in how they pursue and maintain outside relationships. Take the time to communicate your jealousy and envy. cutting off your partner(s)'s experience just because you "feel" some way about it is not okay. Opt for parallel polyamory, garden party, or don't ask don't tell DADT dynamics to avoid dealing with metamours you dislike. If you feel like you cannot trust your partner to make decisions for themselves - break up. they are an individual and do not need to be chaperoned in non-monogamous relationships. If there is an issue where a partner is experiencing abuse but refuses to leave that metamour - a veto probably would not work anyways and it may be best for you to separate. Vetoes should not exist to validate someone's importance either.
One Last Thing
Vetoes are not limited to individuals dating separately. They can be done within a triad+ relationship to affect those within the dynamic or outside. Vetoes could be used in BDSM dynamics where there is TPE (total power exchange) but any outside parties should be aware of this so they can decide if they wand to consent to a relationship with someone releasing their power.