People are probably more inclined to get divorced and have more children with different mothers and fathers than they were 10, 20, or 50 years ago. It seems very popular and modern these days to have things split up and have different views on fairness, unfortunately, with the understanding that people can grow apart.
If you want a relationship that works long term, you should have a joint economy and be married (in my opinion), so then it’s 50/50, and you should view bonus kids as your own. People should get married more often to sort this kind of thing out.
But then again, it can feel strange if someone has 3 4 kids already and you have one of your own, and maybe you’re expecting one together. Even if you get married and aim to treat all the kids as your own, no matter how it was at the start, you might still want to prioritize your own children.
Especially when it comes to bigger things like saving, driver’s licenses, and inheritance, and if one person earns a lot more.
Ideally, it’s 50/50, but I think it’s a very complex issue depending on the circumstances within each family.