Long used jokes sometimes don't work

There is a chapter in every management textbook about self esteem. I explain that our self esteem is impacted by many internal and external forces. I then tell the following story:

As a child I watched the show Brady Bunch. In one episode Peter Brady thinks he's adopted. Any kid watching the show has to wonder if he, too, was adopted. I asked my mother and she replied, "Kid, if we could have picked do you think we would have picked you?"

The joke has always gone over well back home. When I told the story today the guys sat in stunned silence. I counted to three... and started laughing hard. Then they realized it was a joke and started laughing.

In the next class one student laughed right away. The rest of the class looked at him like he was horrible. Then I started laughing and they realized it was OK.

Update: All 3 female sections laughed right away.