OK, first up - there is no one "right way" to do poly.
If the rule in place is that the married couple can have sex without the third but that if the third is involved then it has to be you as a couple, then so be it. That doesn't make it not-poly suddenly - if it works for everyone involved, then that is absolutely great.
HOWEVER - the mere fact that they went ahead and did it indicates to me that it's NOT something that works for them (and like I said "I was drunk" is never an excuse) - if they did it, then they did it because they wanted to.
I would strongly suggest that the three of you sit down and talk about this, and revisit it. Maybe that "we only play with her together" rule needs to be changed.
"Listen, or your tongue will make you deaf." - Native American Proverb