My two major examples of infidelity with 2 partners - one was a lover many years ago who just up and left and that was the end of it. He basically left for a better economic situation (people might be surprised at how often this crops up). I never heard from him or had contact again, it was just I'm leaving, goodbye. Took a long time to recover from.
With my spouse, it was online dating apps and a longtime EA with an old GF. Nothing physical. That it was nothing physical was an immense help. It put it in a different category for me. Fantasy vs a sustained and contrived reality. He had tremendous stress in his life and this was a fantasy way of dealing with it. Not healthy or helpful, but understandable to me. I hit the roof, which also helps because I think so many people take it really personally and think it means something bad about themselves, or they get anxious, depressed. I get flaming mad and let it all out. I also recognize that even though it hurts personally.....that's not the kind of thing that reflects directly on me. It's about him and his problems or needs. I'm not someone who does take a lot of things personally.
I do have a lot of lines in the sand, and to me they are pretty firm. I would not tolerate anything involving a friend or relative - you're GONE, BABY, GONE. To me, that's the worst kind of thing anyone can do - it's not just about the sexual infidelity, that's such a basic disloyalty and contempt on the part of both people - a sibling, a best friend, whatever. To me, that really is unforgivable. Also, cheating in the family home, someone who uses our bed to cheat in, that is unforgivable. Again, it's such an expression of disrespect and contempt to me, such a violation of privacy and the basic marital bond. I could not get past that. I would not forgive a serial cheater. These are solid lines to me, I can't imagine accepting someone back after these. I just would not. There are other things that probably would be true, like physical abuse of me, or the kids (I don't have them, but if I did, treatment of the kids would be paramount). Beyond that, it would depend on how they were conducting themselves....in general, I'm not a supporter of recon, I generally believe in moving on, life is too short to keep patching a life raft with holes in it. I feel that people show you what they think of you, what you mean to them, or what their priorities are, and I pretty much take them at what it seems to be. I don't generally believe people change - they can - but most don't, so when I see a lot of evil shit....I just assume that's the way it is, that's the way they are, let me go my own way. I can only tolerate as much as I'm willing to tolerate....and that's not that much.
I would also not tolerate a long term affair, if I found that one existed. If a guy is going to continually put sex, energy, money, emotions, into another woman, sometimes for years....that just means to me that our relationship is dead at the core anyway and why should I waste myself on someone who would treat me like that. Like a harem or concubinage or some such. Go have your harem somewhere else, I wouldn't waste myself on someone like that.
Also, I would not stay involved with someone who got an OW pregnant. Never. That's a firm line, and again, happens more than we think. It's the usual disrespect of cheating compounded by real carelessness and if a child is born, someone who's going to STAY in your life forever. You can't just disregard a child.
The most forgivable things to me are one timers that really are purely sexual and don't seem to involve a lot of emotion or complexity or disrespect to me (aside from the lying). I understand everyone has sexual feelings for more than one person, we just have to keep control of them, but sometimes we lose control. I might forgive a mainly sexual affair (unless I get an STD - then, LOOK OUT!) if it's an ONS or one off because it's really THEIR problem, or their issue. I don't really think it reflects on me, unless we have a real sexual problem in our marriage - no sex or bad sex. Especially if he just comes to me and says, look I screwed up, I did a bad thing, here it is, please forgive me. I might very well say the hell with it if it IS a one off or ONS and not a pattern. If it's serial cheating or hookers, or Ashley Madison....then it's a no go....continuous cheating is DISRESPECT and that's my bottom line, if you really keep disrespecting me, treating me bad, lying to me over and over so I can't trust what you say....why would I want to continue in that. I can do better elsewhere or I can do better just on my own. I won't keep eating a shit sandwich, I prefer STEAK!