Why makes people TTC when not married?

All my life I knew I would have children one day after I got married. It just how things work. I know accidents happen, and good parents don't have to be married to eachother. But what makes women decide to PLAN pregnancies with their boyfriends? Serious question. I'm a firm believer in "if you wouldn't make her a wife, don't make her a mother". I know marriage might just be "a piece of paper" to some, but it's a real commitment, just like having kids. What are your thoughts?