I feel so guilty about this

I have a baby boy who I adore and love more than the world. I wouldn't change him. But I can't help feeling a bit jealous about people who have baby girls. I thought this feeling would have gone by now. 
My son is my world and I would be happy to have another boy but I would want just one little girl in our family. It's not that I don't want a boy I just really want a girl. 
I feel so guilty for feeling like this. Does anyone else feel like this?