The REAL problem?

This is just my interpretation, but I think the biggest problem between David and Diana wasn't that Diana had sex with Gage. I found that there were two other problems Firstly, David couldn't provide for he & his wife, so he essentially had to pimp out his wife to another man (who was more successfully financial) and thus felt emasculated. Secondly, Diana wanted David to be possessive of her, and he wasn't. These issues were evident before she ever spent the night with Gage. The fact that David had a change of heart, yet she went through with it (like they agreed), then made it worse from both their perspectives. Diana's hates that her husband actually let her sleep with another man (she wants someone to stand up for her), and David felt inferior to Gage (he accuses her of wanting to sleep with Gage before he even made the offer, constantly questions her about enjoying it, etc.).

That was just how I saw their relationship, it wasn't the sex between Gage and Diana that ruined their marriage, it was a lot deeper and more personal than that. I think the big issue was mainly that Diana wanted, and expected, David to flat out say no other man would ever touch her & she was all his, and he didn't. That alone, even if no sex had actually happened, would be enough for Diana to start questioning their relationship (I certainly would if that happened to me!).