Venerable John Paul II on marital infidelity:
“The strength of such a (mature) love emerges most clearly when the beloved stumbles, when his or her weaknesses or sins come into the open. One who truly loves does not then withdraw love, but loves all the more, loves in full consciousness of the other’s shortcomings and faults, without in the least approving of them. For the person as such never loses his/her essential value. The emotion which attaches to the value of the person is loyal,” Love and Responsibility, n. 135.
I just stumbled on this site. Lots of wisdom here.