Man this might be easier said than done, but think of it like this: it’s a kinky nasty perverted thing she did. AND: it’s kind of hot! Your girl went out and took somebody else’s cum. She’s a freak! And it sounds like she still takes yours, and loves it. Obviously it was kind of a rash decision made under stress, but in the end? It’s just not that big of a deal. Think of the hot kinky aspect of it, and if you still like her and want to stay together, focus on that part of it and move on. She seems to care a lot about you and not at all about the guy she acted out with in a one off.