If it was my partner, I'd forgive him, but that would also be the end of the relationship. At this point, it's no harm, no foul. If a partner wants to behave like a single man, he shall have his wish. Sleeping with someone else is never an accident. It's an intentional decision. A man (or woman, for that matter) may regret that decision, but that doesn't change anything. Hard situations are when your true character shows.
I would feel differently if there were extenuating circumstances, like if the other spouse had Alzheimer's or something, but not under normal circumstances.
When I was single, I once turned down a woman, who---when I saw her about 3 weeks later, after she said "Hello" politely--- hammered her fists on the table and demanded to know why I wouldn't go to bed with her 3 weeks prior.
I told her that she was certainly attractive, but that we would have had a great time for 3 or 4 months and then it would most likely ended unsatisfactorily for one or both of us. We ran into each other a few times after that---once when she dropped into a bar at which I was well known when she was 7 months pregnant (and still single). We talked, she left, and I explained to a few of the employees that it was indeed not my child. (She had previously worked at that bar and restaurant, and she returned a few months later with her black boyfriend and her "black" child.)
There were others.
Then there was one girl that I had known for about 10 years that had come moved back in town after her divorce. I helped her move into her apartment and she and I watched the sunrise from the balcony while drinking Piesporter Mosel (back before Germany changed its wine classification.)
(I hardly would call that night spontaneous, we had both been imagining it for at least 9 years.)
I was single then as well.
I mention that history because I am an man, I have always been faithful to my wife; and I have never strayed, but not for lack of interest on the part of other women.
My value system does not allow that on my part.
But while I think "cheating" is serious, I consider it intellectually incomprehensible and yet psychologically understandable.
And I don't know offhand of any marriage vows that say "for better or worse unless you cheat."
So finally my point---It does not have to be a "deal breaker." Of course, if you deem it so, then you also risk losing a relationship or a marriage that does not have to be lost because an arbitrary position has been taken on such an event.
LOL I am a very non-judgemental person, ( well unless it's about Trump!) but... it is also true that NO man is helpless.. weak yes but helpless no.. they can say no, thank you, I am married, or with someone... So if they don't say no then sorry this IS their fault.. I mean if we are on a diet and someone gives us a huge sticky choccy cake do we have the power to say no? Yes, we do.. weakness and greed are at fault when we don't say no.. it's the same with someone who cheats even if the lady throws herself at the man..
On some, or even many levels the man wanted to cheat.. I know its hard to say no.. but he made that decision or failed to make the decision to stop it..
I don't think that cheating is necessarily the end of a relationship, but that would depend on circumstances and what explanation and how sorry the person was.. it might be difficult to trust again after that..
I don't judge those who have cheated but again we have to rememeber that cheating by definition is not a good thing, not something to be proud of.. I have talked to many men, and women who cheat or have cheated and what really needs to be remembered is.. that even if the intention is that the one cheated on never finds out, it IS still lying to, fooling, hoodwinking an innocent person..
I am all for open relationships, (though in practice they are not as easy to manage/ non-dangerous as you'd think) where there is honesty - because then it's not *cheating* but when one party doesn't know what's going on ... then it's lying and dishonest.
~ 
Art Lover 