Why is having children or getting married almost seen as a necessity?

Why is it that when I say I don’t want children I get told “Oh, you’ll change your mind”???? I get I’m 19 but I literally hate the idea of having kids. It doesn’t seem beautiful to me and I do not want to take care of them! It is for purely selfish reasons too. I do not want to be bothered with having to look after another human being, plus I feel like with me having depression and all kinds of other issues I’d either pass it to my child or it would just make me a shit parent. I would rather save someone else from dealing with that because I wouldn’t handle parenthood well at all. It just makes me feel gross thinking about carrying a child in my stomach for 9 months. Don’t even get me started on childbirth.

Also, why is the same said for people who don’t want marriage? Why is “You’ve been together for 7 years and aren’t married yet? How?” a question? Why can’t people just live together without making it legally official? I don’t have plans to get married. Never really thought of it but if it happens, it happens I guess. I’m not as opposed to marriage as I am with me personally having kids.

I just feel bad for those who choose to do neither and people look at them like something is wrong with them. I don’t hate children just wouldn’t want them of my own.
I suppose since these things have been the norm for EVER it’s going to be hard to make it just as normal to do neither.