A childhood belief about marriage

When I was younger I started to wonder how my mom can dad can just get dressed and undressed in front of each other so nonchalantly even though they were opposite genders. I asked my older sister, and she told me that when people get married, they have to go into a room and stand naked in front of each other. This made me nervous to get married. I also figured that if two people got married and they had children, like in The Brady Bunch, that the children would have to do the same thing with their new step siblings.