It doesn't always happen, but sometimes you need a bit of the sharing, or at least some discussion of openness before the cheating comes in. The gf/wife gets a taste for it and wants more than the bf/husband is willing to go for, or he dangles the idea in front of her and gets her interested but then backs out and she doesn't want to back out. I think in this game it could have been earlier, but we're at that point about now, and I would suggest that dancing with Luke's dad would offer the first actual cheating opportunity.