I don't really know how to respond to this myself. I have many things going through my head, but I don't know if you guys have played separately in the past or what your boundaries and rules were prior to this incident.
I think Spoo and EvilMJ said it pretty well, and your communication is definitely in need of some improvement and overhaul.
In our situation ... neither of us would ever go to a dinner or meeting with a single or even a couple for that matter without the other.
I consider it extremely disrespectful to you that she indicated, in so many words, that her intention was NOT to have sex with this guy, then she did. I consider it absolutely disrespectful that she called you to tell you that there was stuff in the freezer for you.
And absolutely without a doubt disrespectful that FOUR full hours went by where they were together and you were home alone??!!!!!!!
I may have read your post wrong, but I thought you said that your late appointment would cause you to have to meet them later .... then it sounds like you were just sitting home alone while they were out? I don't get that part.
Yeah, I consider it cheating if she led you to believe that they would not be having sex, then did without checking with you first to make sure it was ok.
And I really don't see where SHE gets off telling YOU that there are no more rules and things are just a free-for-all from here on out. THAT is TOTAL disrespect, IMHO.
Y'all need to step back, pull out of the lifestyle (if she thinks she can do it) and re-examine your priorities. Because if her priority is the lifestyle and not you and your feelings, then you have a lot more serious issues in your marriage than the lifestyle and the way it is handled.