This is sadly normal in Japan. Cheating, lack of communication, the toxicity. Incapacity to conciliate spousal intimacy with children.
The author is just putting what happens in real life on the page, just like in Genshiken. You don't have to like or endorse what the characters do, and besides Madarame has always been a spineless loser. This is hardly out of character for him. Reading and enjoying this manga does not mean approving of him cheating, and in fact i'm curious to see if he'll ever get his comeuppance and how the rest of the cast will react to it.