You are correct that it's lust that causes her to cheat. I disagree however with the assertion that Patrick "dangled the forbidden fruit too much in front of her". Lots of men encourage their wives to dress sexy and to act in a sexual manner for her to loosen up and for his enjoyment as well. That doesn't give her a license to cheat though.That's the thing, it's kinda true. Cheaters (women in this case) will often blame the partner for "making them" cheat, by not fulfilling all of their desires and whatnot. Truth is, there is never any justification for cheating on your SO, regardless of how "open" you two might've become.
Though, in some twisted way, Emma is right in that Patrick dangled the forbidden fruit too much in front of her, and then told her she won't be tasting it after all.
It's called lust, and that's why she cheated.