Marriage, it is recorded in the scriptures that the Buddha explained how to be a good husband and how to be good wife. There are five or six distinct points that he would say was a responsibility for each one of them. It was largely based on the traditional way of Indian marriages at that time, but it had an element of compassion and sharing. Two specific items, one for each that was exactly the same was that the husband should give the wife gifts and the wife should give the husband gifts. Giving to each other was something he was teaching, not just taking from each other. Often when we go into relationships, we want. Can we go into a relationship where we give?
The Buddha did actually explain ways of being a good husband and wife. With adultery in particular, it's an absolute "No!". It's not to be done. Why is it not to be done? It's not just because it's a vow, a "shall not". It's because it is out of compassion for yourself and for others not to have adultery, to be content with your wife or your husband. As well, to be content not to interfere with someone else's marriage.