I notice a lot of posts from ex-forever-girlfriends are about how they were with a man for 6 years without marriage but after the relationship dessolved the guy found another younger woman and married after like 5 months. Why they did not choose to marry the girl they knew for long and yet marry a new random girl fast?

I'm a man, but this seems a valid question from these women.

I wonder if the reason is that men have a category for forever girlfriends? What kind of women are included there and why they don't seem to get commitment from men even being open and trustful to them? Is the difference fundamental?

I personally don't find any issue with this because marriage is meaningless in the sense you get sex, commitment, children, etc. without it, it is mostly symbolic and women put too much weight on it, but I'm welcome to be proven wrong.