You need to understand that in Japan marriage is seen as mostly a duty and a contract. So this depiction is not something odd. It would be easier to understand if you think about it as marriage in Edwardians times, where people married for status, wealth and appearance. If you think for example in pride and prejudice where Elizabeth Bennett's friend is regarded as a spinster for not being married at the age of 25, in Japan is pretty much the same.
In this manga you can see it with the guy who goes to sleep to his co-worker's flat because his wife doesn't let him in after a certain time. In a flashback you find out his wife is cheating and he's regarded as a "pay packet".
all the cheating husbands really didn't do it for me though. I can understand the two have been friends since HS, but really once one of them got married, the sex side of the relationship should have stopped unless it was with the wife's consent. It wasn't really presented in that way. At least the other guy was good enough to cut off the sex friend relationship when he got a lover and committed to him.