Some people think too big a deal is made out of unfaithfulness. They must, since so many people do it.
Countries like France and Italy institutionalize it.
But if we take it out of the emotional realm of marriage and think of it as any other contract, we can see it in its true light. If we are rationale, we can liken it to other mutually beneficial partnerships.

Such as ice dancing. Or button manufacturing.

If you invest your life in manufacturing buttons on the strength of joint goals with another investor, and then find you’ve been blindsided by a misrepresentation of interest and participation by the other partner who lets you do all the work, but takes half the profits while neglecting your business, and consulting with another button manufacturing company across town, you can get really mad.

The crux of it, from a strictly practical, non-emotional standpoint, is that in marriage, like business, if you’re goofing around, off somewhere else, it's inevitable that you soon may be putting a closed sign on the door.