How did this even happen?
Clearly she did it for the exact same reason you did it: it turned her on. It's possible that she too wouldn't have cheated on her husband, but you both enjoyed your little flirty moment there.
She was drunk.
This fantasy is getting very old. Things are even more boring for you in Mommy's basement than I had imagined. I'd suggest you actually date a real live woman. That will keep you occupied.
You don't stop being human just because you get married. This works the same for both men and women. You can make a promise to only have sex with your wife or husband, but you can't turn off your feelings of desire for other people. Most of your story is boring and rather predictable. You were turned on, so was she. The only surprising part is that she was demonstrating physical affection while sitting next to your wife. I can only guess she was really REALLY drunk to the point where she did not care about the possible consequences of making your wife really really angry. As you are not the cheating type, there is no reason to analyze the situation further.
Just because she didn't want to pull her hand away... that's why she did not. Maybe she thought there was a possibility for a threesome? Who knows?
take a glass of milk to your room drink the milk fap milk yourself into the glass that's what i've been doing since i started beating it