
You are less prone to covet if your spouse is your standard.
Consider for a moment all of the people that your eyes see on a typical day. Consider those people you see on television, while watching a movie, walking at the store, interacting with at work, or on social media and pornography you are viewing. How many of those people do you entertain sexual thoughts about, even briefly?
Now, consider your partner. How can they possibly ever live up to that standard? How can they be so many different kinds of people of different ages, races, appearances, and personalities at one time? They cannot. The standard is impossible.
This is the problem with a standard of beauty. We change, especially as we get older. A standard of beauty is always increasing as we add the best features of more and more people to that standard.
Much of the world is like gravity seeking to pull us into coveting someone other than our spouse. When we fall prey to this gravity, it drags us and our romantic relationship toward death and hell. Coveting is also breaking the Tenth Commandment which forbids us from lusting after anyone other than our spouse. The first step to coveting someone else is comparing our spouse to a standard, which is nothing more than someone else that we wish they were like.