While bad relationships are probably the most likely cause for a spouse to cheat, it is not at all uncommon for an affair to occur even in a loving relationship, especially after having been with the same partner for many years, and all that newness and luster has worn off. It is very common in the workplace, where a woman spends more time of her day with her work colleagues than with her husband. It can start out as a simple friendship, and soon enough emotions begin to develop, and then that long lost excitement is rekindled that we all have when beginning a new relationship. I have seen this happen countless times in my place of work over the past 20+ years.
So you see, it doesn't have to be the typical ridiculous scenarios that are manifest in most horribly written NTR games. Besides, I don't even think it is possible for Roman Hume to write anything badly. It's not in his blood.