This manga seems kinda cliche for some, but it's not that different than in reality.
There was a common mistake, shared with all 4 main characters, i.e. smitten by love or lust before making an important decision (marriage).
Michi: typical (career) woman who don't mind to be a mother and full time housewife.
You: typical game otaku that detached from reality most of the time. He don't really care about having a kid. Even if he did (then he's a dimwit), he still don't realize that women have a very limited (fertility) time/age to conceived a baby.
Niina Makoto: typical man that willingly to carry his marriage even if his wife a full time housewife.
Kaede: typical ambitious career woman who don't really care about having a kid.
Now we understand that all the MCs should see these "flaws" in their partner before marriage. But the "love" blinded them. Ones should expect that their partner will never change from the get go. If he/she eventually changed, then that's good for them. If not or even if gets worse, then they already expected that.
Some of us was thinking that why cheating seems easier than divorce? The reason is simple, we're human tends to craving for dopamine rush (the excitement hormone). It can be through games, a fling/cheating, or even adrenaline rush from extreme activities.
We should realize that soon or later the "sparks" will gone, the only one that should always left is commitments. Just like respect, commitments are mutual, works two way; And it's not granted, but need to work at.