Within a marriage, it is debateable what is "loyalty."
I see it as someone is looking out for my needs versus their own needs. Is my spouse's need more important than my need? Well, yes, sometimes it is and sometimes it is not. If her needs are always more important or very often more important, than we do not have a "we." This can get sticky with entitlement thinking. I am entitled to my spouse's ___________. That doesn't work well. Instead, we express our needs and preferences. The spouse takes those into consideration and prioritizes. If my preferences always lose, then we have some conversations.
Many of my clients say they do not feel like a priority to their spouse. The job is first or the kids come before the spouse. Rounding out the top items are Bowling and Golf, and the in-laws.
How else does someone show that they are not loyal?
Being unfair, lying, cheating, being cold and simply looking out for themselves. Remember, we are partners.
Someone accused me of being unromantic (not my wife) about this. "Don, why would anyone ever get married if it is this tough and this fraught with turmoil?"
Here are several benefits of a "good enough" marriage:
Mutual trust allows for greater partnership
Deeper love allows friendship to blossom and intimacy as well.
Someone with whom to share the duties
Someone who listens
There are more. Anyone want to list a few?