In the triad I was in, I did not believe we had to be together. It was initially a rule but we dropped it pretty quickly. It was just to much pressure.
That said it isn't uncommon. I have heard of triads come up with really elaborate plans to figure out when the "group" is ready. 3 candles in the hallway, if all 3 candles are lit, game on...if not, no one is allowed to have sex. Very strange pov from my opinion, but hey whatever floats peoples boats.
Sex is always best when there is a level of selflessness. Especially with all parties involved. Otherwise, it just isn't the best sex it can be (well unless you are into selfish sex...but thats a whole other beast)