Because we got a dikk nikka, it ties with the same reason were expected to risk our lives to defend our women and family, cover the majority of the financial struggle and so on.
This doesn't really explain anything about why it's worse when women cheat. If you believe that the expectation of men to risk our lives to protect women and family entitles us to being able to betray the women we're in relationships with, why don't men just let women know up front that they will have sex with other women?
Why are men calling themselves men and then moving in secret like women?