The marriage as a whole. I see accurate representation of expected monogamy/polygamy or fidelity/infidelity (WRT sex) as just one aspect of many that may cause marital problems. What about a husband that expects the wife to be a homemaker when she is unhappy in that role, or a wife that expects the husband to provide all financial support when the husband is unhappy with that. There are many reasons marriages fail, and I think focusing specifically on one of them may not help as much as making people more aware of marriage expectations altogether.
As for the rest of your comment, I would be happy if marriage as a whole was less strictly regimented by society, but I understand the reasons it is. Marriage is the building block of families, and families are the building blocks of our societies. We optimize for successful families that produce well adjusted offspring. The problem is that I'm not sure exactly how we are defining "well-adjusted". If it's primarily in relation so society and culture (it is, the question is how much), it may just be self selecting. That's not necessarily a way to make things better, just to perpetuate the status quo. I just don't know enough to make a call.