Gatekeeping "marriage"

[CurlewKate]

Something I have noticed happening more and more-someone will post an issue about their step children or in-laws and a poster will say "They aren't your in laws/step children-you're not married."
As if you should have no involvement with your other half's family if you're not married.
Is this a widely held belief and if so, why?

Well, I think there is a difference between married and not. Part of the 'problem' (for want of a better word) is that people refer to their boyfriend/girlfriend of 20 minutes as their partner, which means people in longstanding relationships get lumped in with the same terminology. 'Married' implies a serious commitment.
I think maybe you are getting accidentally put in the same category as others who talk about their step kids, when they mean their boyfriends children, who they've known for 3 months!