This is an absolutely ridiculous argument.
Of course it's not. It's very logical. Too many people don't think things through and are swept up in the emotions of children and a relationship.
You are talking like they are two separate autonomous people making decisions about their separate autonomous lives. They are in a partnership, choosing to have children together and raise a family, with an agreed verbal understanding that they would get married.
Well, basing your entire financial future on a "verbal understanding" is a huge risk to take. OP took the risk and it didn't pay off.
OP has been clear that the decisions she made were in that context, and were to benefit the family, and clearly have benefited her partner. These were decisions made as a family unit.
Were they? Or did OP do what many women do, which is choose to take time out from work to raise her children - it's funny how you never see an unmarried man going part-time once children come along.
And most people trust their partners when they make agreements. We all trust out partners everytime they say they are going out that they are not out having unprotected sex with other people. Relationships work on trust. OP trusted him. She made decisions believing they were getting married and she would have protection. Blaming her because he has turned out to be untrustworthy is utterly unjust.
Why shouldn't she take responsibility for her actions? Basing your entire life on the verbal promises of someone else is silly, no matter how much you love them. That's not to say "you have to get married" but be sensible. If you don't have that form of protection, then don't take time out of the workplace and give up your financial independence. Men don't do it, so why are women so keen to make themselves reliant on someone else? I don't get it.
The argument, well she didn't have to, if applied widely means absolutely no-one has any obligation to keep any promises or agreements they made and more fool anyone who trusts absolutely anyone else on this planet.
Well, nobody does have any such obligation, do they? If you choose to have children without the legal protection of marriage in place, then you need to protect your own interests. Nobody else will do it for you.