I can explain it easily. Exclusive Pair Bonding is not really part of the human mating instinct. There is an instinctive basis for male infidelity and for cuckolding. Both increase the chances for the male and the female to pass their genes to the next generation. And unfortunately, rape is also a part of that instinctive human reproductive strategy. That's just how life works.
The cultural bias toward monogamous pair bonding is a fairly recent innovation, when you look at it in the context of the entirety of human evolution and history. So it really shouldn't come as much of a surprise when either party strays, even good relationships.