If my husband and I are arguing, we're likely not having sex till the conflict is resolved. That's two of us in that relationship. If one or both of us is seeing a secondary and we are arguing, there isn't any running out the door to have sex with secondary partners, avoiding the icky when we have something we need to resolve. That's two of us plus one plus one.
If there is three and all are equal, but two of the three are arguing, why would it suddenly be okay to dust off the emotional work of resolving the conflict so any can get laid? If setting it aside to get laid is so okay, why exclude any one of the three?
I'm imagining a developing triad situation we're exploring and how well it would go over if my husband and I had a conflict and before we resolve it, I instead begin intimacies with our third. I asked my husband how well that would go over and he said it would be hard to not feel like the conflict didn't matter to anyone but him and didn't affect anyone but him in that situation. A bit of a "you're on my nerves so we're putting you out in the yard for the night" feeling and it wouldn't go very far to keeping the third from being dragged in the middle in a negative fashion. Of course I agree with the if you want something from all parties you ask it of all parties, but if you're mad enough to not want to enjoy intimacies with one partner and they and the other you're not in conflict with go off and fuck it seems a cold way for three to be on the same page.