Why not get married?

Da
Hi!  I do not mean this to insult anyone, just genuinely curious.  Before I joined here I always assumed people who had children without being married had Accidently gotten pregnant.  Now I am realizing many many people are actively TTC and even at a very young age, unwed.  Just curious why you wouldn't want to be married?  And I don't mean have a big fancy wedding, just officially be married, share a last name, legally be family, etc..?