What I'm seeing from my own three daughters (in their 20s) is that they have no desire to get married, possibly ever. They don't want children, either. Things have changed a lot since we were younger.

I think the trend you mentioned in your OP is becoming more common with people of all ages. Two of my sisters (one is 47, one is 50) are divorced and have been with new partners for many years (one 7 years, one 17 years). Neither of them have any plans to ever get married to their current partners. The don't see a need to.

Some of us do still believe in marriage though. It's definitely not a goal for me, but with the right person I totally would.