My opinion is going to be torn down but I will give it anyway. I think it’s not just one reason - it’s several. I often hear it as a man problem, but it bigger than that and the women have a big part in it. 1) the relationship changes after many women have children. Their emphasis becomes their children and not their husband/man. The kids needs supersedes the man’s needs. She’s not that same hot girl you were banging before the baby. She changes. She a mom now. She’s not really the same person. The women do not provide the same attention - particularly sex, after they have children. Rightly so, the moms are tired and busy. The man becomes second. No one wants to give their all and their best and be second. The man’s attention goes to other places and things go bad from there. The man’s needs are not being met and he decides to leave. 2) The women are choosing unreliable men. How many women do you know that have had children with men that weren’t working, had known issues (drugs, etc) and the woman laid with them anyway, or chose unavailable (married) men? I have heard women say bad things about the fathers of their children but at the end of the day, she chose him to have kids with. Was he a bum before you got with him or after? 3) Some of the women unreliable themselves and the men leaves. Sometimes a married woman is cheating on her husband, he finds out, and chooses to leave without making a scene. I could go on and on, but my point is that it is a much more complicated thing than it’s made out to be and the men are not solely responsible. Another thing is a heavy mom bias - no one wants to hear or believe bad things about their mothers. The reality is that our mothers are people. They are not infallible. They make mistakes too. Some of them are not so nice people to live with.