As someone who has been married over twenty years, I feel qualified to answer this. My wife has not, to my knowledge, cheated on me, but I think I can speak to the question even so. My view is that there are some kinds of “cheating” that I could forgive, but others that would be harder.
I think that it would be easy for me to forgive my wife for cheating on me purely as a physical thing. We have, after all, been monogamous for many years and it would be very natural for her to wonder what it would be like to be with someone else. It would be natural also to want to prove that she could attract another man even at the age we have now reached. I think I could readily forgive her if she had a casual affair as a way to experiment or to reassure herself of her attractiveness.
I think it would be much less easy for me to forgive if she entered a deep emotional relationship with another man. This would feel much more like she was trying to replace me. The hurt would be much greater. In addition, I suppose it would be much less clear that she would want to be forgiven. She might just want to leave me in that situation.
Overall, I hope I would forgive in either situation. We have had too many years together for me to want to give it up out of wounded pride. I would hate to put my children through a divorce and a broken home. I would hate to put my parents through it since they love her as a part of the family. I would hope that I would swallow my pride and forgive her so that we could keep our family together and, eventually, regain the strong relationship we now have.