I wouldn't have liked that situation either. If there is nothing going on, then they should have: 1)had a drink or two in the kitchen/living room area (out in the open and not in the "man cave"), where they can be seen and/or heard by the boys if they should be on their way out; 2)gone somewhere public such as a bar or café where they were out in the open; or 3)should have waited until you were there to meet up. This kind of situation, i.e., drinking and partying by themselves could lead to something else.
The meet up may have been innocent, but it doesn't provide a good example to your sons of how people should behave when a spouse is not around. To me, both of them were disrespectful to you because I'm sure they both knew how you felt about this type of behavior.
I certainly would stand by my boundaries and she wouldn't be allowed in my house unless I was there, no matter whether I was the third wheel or not.
Just my two cents.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
