Outside of death very view things are "inevitable". The goal is always to (find someone who wants what you want and naturally agrees with you on the major things). Having said that; (When we change our circumstances change.) In other words it's important to keep doing all the things that attracted your mate and caused them to fall in love with you. Too many people subscribe to the "treat the (new) better than the (tried & true). They bend over backwards to win over someone at the start of a relationship and once there is an emotional investment they slowly stop doing most of those things while expecting everything to remain the same. When we change our circumstances change.
Cheaters usually are looking to hold onto what is good in their primary relationship while addressing other needs on the side. Not many people are willing to throw away a relationship that is 75-80% good. They'd rather take a chance of not getting caught cheating than to run to the court house to file for a divorce. You really have to stay on top of making sure each person knows the other person's (true) needs, desires, and wants.