The Catechism gives a great explaination, I’d enter here and read the entire chapter:
A real life example:
Imagine a married man. His wife fixes supper every night and has it on the table waiting when he gets home.
One night he decides to go to have a couple of beers with work friends. He shows up 2 hours later than normal, did not call his wife to let her know. She is angry with him, but, they make up and all is fine.
Same man, one night he decides to go after work and spend 2 hours in a motel with another woman. She is angry with him, this will be a deep wound to their marriage, a quick apology will not heal this wound.