I made a mistake before. "Monogamy" controls male aggression. Marriage is just a legal arrangement. It's primary intent was to protect property. If you don't plan to have kids or transfer property why marry. Just let your yes mean yes. If a woman and man stay together long enough the state grants them the same property rights as a marriage.
Beyond that any two people who live or work together have numerous transactions that define the relationship. It goes without saying that the relationship that RESULTS from a marriage agreement can take any shape or form agreeable to both. When they don't agree anymore on something big or on a lot of little things either of them may decide to end the relationship.
Cyber-sex on top of a lot of other discontents might be the straw that breaks the camels back.