Aaah, but it takes two to tango. You seem to assume that the onus is all on the partner who wants to persist in maintaining both relationships, as if the person who is actually saying NO isn't also responsible for deciding what it is they are not consenting to. If partner says 'no, I do not consent to you seeing other partner, but I will have sex with you anyway even though I know you're still seeing him/her', then it does muddy the water somewhat, no? This is why in that scenario, they are not actually withdrawing their consent to participate in the relationship at all….they are trying to impose a permission-system on their partner's OTHER relationship.
Unless I have explicitly agreed to do so (and the only context I can envisage that is via a power exchange/BDSM type relationship) I am not responsible for maintaining the personal or physical boundaries of my partner. That's their job, as it is mine to manage my own. In the advent that I become upset with being in an open relationship, my first step will be to explain my feelings and see if there is an easy way to resolve it. If my partner is not in agreement, then me withdrawing my consent HAS to mean me walking away from my relationship. My partner is not being unethical for wanting to continue what we had, but they simply cannot be in a relationship with me without my consent. If this is a scenario where my partner wants to open up our relationship from being closed, the same logic applies. If I do not consent to that proposed change in my relationship, then we are no longer in a relationship. You might want to nitpick a little and say in that scenario then it should be my partner who is taking the stand and bringing the relationship to a close because it is they who has the desire to change the parameters, but I take the view that I take care of me. If not continuing a relationship is the best thing for me, then that's an action I should take to maintain my own boundaries. If partner is no longer able to partner me in the way I desire, then I should be free to find someone who will.