talking about harem cheating: that is a good question to rise, if he can have multiple wifes and then cheat outside, can't he solve everything by bringing the other in?
so cheating should have a different weight for those situation, but i really don't know, not like i was raised in such culture.
there is a difference that on 2 way only relationship cheating hit really hard because most chose the ultimate 'me or they', a choice that can mean break up/broken hearts,
while harem could be 'stop or make it official' or 'too many child, cut his thing'. i don't really know lol, it is hard to imagine another culture.
in the end, i still think it is less unlikely to arrive to the ultimate that would for sure mean breaking up, since there is another option.