Is it a red flag if a man hasn't married the mother of his kids yet? We're talking people in their mid-late 30s?

It's more than a red flag. It's a clear indication marriage is not likely. Given that reality, it is up to her whether to seek someone who is available for marriage.

it is no flag it is how did the mother accept for a kid without marriage and did she just realise wow

Grow up

Where I live you don`t have to get married. It`s not important so it would not be a red flag. If however laws could only protect you financially (assuming a house that they both purchased was in his name and she would have nothing if they separated) then it would be a big problem.

Some facts are just that. He didn't marry her. He isn't asking now. Facts are facts. There is always much more involved than the simple questions we hear here. If he doesn't live with you and never has? He's not going to.

Yes! It is much better to marry with the mom of kids.

The red flag is the fact that they have kids. Those two were not meant to be married, and CERTAINLY not meant to reproduce...

Wedding can be expensive, so for older couples on the poor side, it can be put aside for years.

Well, yeah, it's not going to happen. If you are happy, let it go, otherwise break up with him.