Why pillion riding a female biker is frowned upon.

Recently one of my friend learnt how to ride a bike.During a meet up we 4 5 people decided to go somewhere. I handed keys of my bike to her and said please you drive I ll seat behind. Suddenly other men with me started laughing and making faces. Even the girl got insecure because of that who was initially excited to drive it. She asked will it be okay for you to sit behind as a guy? What will people think ? Why is this even a thing ? To clarify: I have sat behind her many times on her scooty. She is a well driver. She did infact drive my bike with me seating behind. And she was happy. My reason for posting was only to highlight how it's looked upon. Cheers!