To find out the sex or not!

Summer
I don't want to know the sex and my mother agrees because that's what she did with me. However my boyfriend says that's not possible you have to know so you can buy things. And I know he knows better and just doesn't want to wait to find out lol.
Part of my reason not to know is that I want a girl and don't want to feel guilty if I'm disappointed and I know if I wait till they are born to find out I will be so happy that they are healthy and in my arms I won't even be able to think about oh no it's a boy.
I only want positive thoughts so please let me know why or why not to know the sex :)