Don’t Forget Your Manners

This riddle, dear friends, I’m told is a lot easier.

A man goes to work. He’s waiting for the bus. Another man joins him. Then a woman joins them. The bus comes, and the men (well mannered) make way for the woman to get in first.

When they are all in and seated, one of the men says to the woman:

“Not very gentleman-like, your son…”

The three of them pause a bit, then burst into laughter.

What did he mean? What’s so funny?

Feel free to post your answer in the comments.