The fact that she was sucking you off in the house that THEY own probably just set it off for her...here was this life she was leaving behind and blowing you there probably just reminded her how upset she was that she couldn't make something work.
Not to mention she probably felt a bit of guilt. Somewhere deep down, she's probably wondering why the relationship broke up...even if it's a simple mutual disagreement, here SHE is being a slvt in the driveway...it probably passed through her head that SHE was the reason the relationship failed, even if she wasn't.
She probably wouldn't have reacted like this if you had hooked up in a more neutral location. In fact, I don't know if you knew in advance that he was up there, but if you DID, you should've brought her back to your place or a neutral location. Not that you give a damn about the guy, but you don't want him coming downstairs pissed off with a shotgun...jealousy can cause a lot of trouble for you even if it's not yours.
Personally, I wouldn't sweat it. What are you gonna do?
All women can be made to "cheat". I swear, some of the behavior I've seen by women...all it takes is the right place, right time, right charm, and often the right drinks. And I don't feel bad about hooking up with a woman when she's drunk...I personally believe in vino veritas...if she's going to cheat drunk, she's thought about it sober and just didn't have the ovaries to act on it.
I just don't trust women any more. I've seen their force of will absolutely shattered by what they're feeling in their gut and in their c00ch.
In the old days, the reason women could stay faithful is because when another man advanced on them, they responded with anger and avoidance...they got AWAY before the man had a chance to communicate with her on a primal level.
Now that women are "equals" and not the housewives they were before, they are EASILY victimized by the clever PUA. As a good salesman knows, as long as they can keep her talking, they have time to circumvent the rational, "I'm in a relationship and can't cheat" response and cause her to have those "confusing feelings".
Maybe that's why I have such a hard time with the idea of being involved in a "relationship". Especially in this day and age, knowing what I know, seeing what I've seen, I know that she'll never really "be mine".