As someone who was cheated on in a relatively brief early relationship in my 20s, my view is probably coloured by my painful experience. But my "partner" at the time was a vain, selfish, narcissistic individual who didn't care one jot about my feelings. I was willing to excuse this behaviour at the time because he was pretty hot in bed and we had some great times going out clubbing and I was pretty naive. But I knew he was lying to me. I eventually called it a day and he implored me to take him back and that he would change his ways. But no, I stupidly gave him a second chance and he did it again. So that was that.
It's nothing to do with Catholic morals - cheating on one's partner is deeply selfish, deceitful and cowardly. You sit down with your partner and try to work things out. If you decide on an open relationship as many increasingly do, then fine as long as you respect each others' boundaries. If not, you go your separate ways. yes it might be painful but is lying and deceiving really a better option? To try to justify getting your jollies behind your spouse's back is a total act of betrayal. Yes, I understand that there are quite a few older men on the classifieds here who married women in less enlightened times and feel frustrated and trapped, but does that really justify cheating with others and potentially exposing their spouse to an STD?
Not good in my opinion.