Why would you bother getting married if you don't wish to create a new family unit?
It's what they choose to do. To have a family, to not have a family. If "having a kid" was the end-all, be-all of marriage, everyone would be getting married when they're teenagers because teen pregnancy is a thing. Marriage is about happiness for the couple, not just spitting out kids. If having kids makes them happy, yay. If not having kids makes them happy, yay. It's all the same in the end.
Yes, but what if someone wishes to pass on their own lineage?
There are many ways to leave a legacy, and blood isn't necessarily one of them. It's who you are, what kind of person you are, what kind of life you lead, what you teach. THAT is your legacy. That has more of an impact that anything in your blood.