A lot of the time it goes like this:
A 20-something couple rent somewhere together, both working. She puts the laundry on because it is no problem - only a small job. He pushes the hoover round, a very 'visible' job, and maybe he enjoys cooking, and she sees him "doing his bit". They get a lot of takeaways because they can afford it. They might be a bit sloppy with cleaning and dusting, or she will do it if she "doesn't mind" and likes a clean home. There is no garden to keep tidy, no DIY to do, etc. The workload is minimal, helped by the fact they are both out at work all day so the place doesn't get as 'lived in'.
Then biology kicks in and she starts to think about a baby. They have the discussions. He doesn't want to lose his comfortable set up and regular sex, so he goes along with it, mostly because it is the 'next logical step' (with or without a marriage taking place, with or without getting a mortgage together). He quite likes the idea of 'being a dad' as a status, and imagines playing football in the park with a mini-me who dotes on him. He might think he understands the reality of nappies and lack of sleep, but actually he has not got a clue.
(To be fair, neither does she.)
Women tend to believe that men 'want a baby' in the same way that women do. Sadly that is rarely the case - the hormones are just not the same. A lot of the time he might quite like the idea, or is at best ambivalent, but he knows if he wants to keep her he has to show delight in her becoming a mother.
Then baby arrives and a nuclear bomb goes off in their relationship. He doesn't know what has hit him, doesn't like how this is all turning out, and tries to cling to their old lifestyle. He resents the way she is always tired, no fun any more, they are short of money and she isn't earning as much, or is paying for childcare, so he resents having to pay for all the treats they used to enjoy. Where did that lovely, carefree 20-something woman go?
So it all falls apart.
If the baby is lucky, the parents work through it, dad starts to grow up, and they stay together.
If the baby is unlucky, dad leaves.
If the baby is really unlucky, dad stays but becomes ever more abusive.