Ladies, what is it about marriage?

Because when women feel that a guy really loves her, she wants to start a life with him, like a family. people usually start families when they are married right?

I think it has a lot to do with insecurity, and I think that insecurity is well founded. Regardless of one's personal beliefs, if a girl wants to get married and you don't, there's going to be a problem. When you replace a belief system with nihilistic philosophy, you're asking for it. Basically that refusal becomes a big "but" at the end of your feelings. "I love you and am committed...but I won't marry you." How can a girl not be disturbed? I'd be worried. You're placing a social ritual above her, and, at that, refusal of a social ritual.

A lot of women dream of their wedding day - they look forward to it and would be disappointed if it couldn't happen. I can understand that. It might feel like failing.

As for labeling a relationship, well I've read way too many questions here about "exclusivity" to be puzzled about why someone wants clarity. Some folks think it's okay to date multiple people at once as long as you aren't "exclusive." So if a girl wants a real relationship, apparently these days she has to indicate that.

As to whether society pressures people...well, heck yeah. That's a norm and pressuring people into conforming behavior is part of society's raison d'etre. Marriage is something that has been passed down through the ages because it works. It keeps the society in order, keeps it whole. Not every social function exists to cause trouble; most exist to create stability.