It's quite easy to walk into these kinds of relationships. Especially when you're young and inexperienced, as it was for me.
Had two children fairly young to my first ever long term relationship. He lived his life as it always had been, work, hobbies, mates etc. Whilst mine changed completely. I looked after the children and did absolutely everything in the house. When he failed to pay bills, I would pick that up too. He barely spent any time with us and even when home, would be in a separate room.
I was very fiery and would argue a lot, but his stone cold indifference to us meant I had to do everything, or it would never be done.
One day, as I was picking up his dirty clothes from the stairs, that he had discarded on his way up to shower... I realised how pathetic I had become.
The next time he had a tantrum and packed his stuff, I let him leave and refused to have him back.
I had been a single mum working full time for several years when I met DH. He was kind and reliable, but even then I laid out very strict ground rules regarding division of chores, bills and how I expected to be treated if he moved in. He agreed. We've been happily married for 15 years with more DC in that time. He is very much an equal parent and as such, I don't resent him as the early baby days were never left to just me as they had been before.
I am 'harder' since that first relationship and don't let myself be treated poorly and absolutely would not accept anything less than fair in the house/childcare stakes. But DH agrees so it's very, very rarely been an issue.
Sadly, sometimes it does take being treated poorly for some of us to raise our standards.