My mom was typically skittish about explaining sex and reproduction to me. At some point she must have explained having babies as something that happened after a man and a woman got married. I have no concious memory of this, but I literally believed that having a baby was not possible until a woman was married. I was completely bewildered when my favorite babysitter couldn't babysit me anymore because, as my mother put it, she was going to have a baby. My reaction: "But how can she have a baby? She's not married!"
A childhood belief about reproduction

others believed things like this
- when I was little I used to think you had to have a baby before you got married!— Supergirl
- When I will little I used to believe that when you got married, you signed a contract to order a baby! LOL— Kriatina
- I thought when you get married you have to fill out a form to get a baby and then you get one.— Danielle
- I used to believe that woman had to pay to keep their last name when they got married!!!!!!!— Imaginative_Kidd
- I used to believe that the man had to be taller the the woman to married her.Anon
- When I was little I used to think that as soon as you were married the woman automatically got pregnant.Anon