Why would they react like this?

I had to go to dinner with my girlfrind and her parents and i was nervous as this was my first time meeting her parents. I really wanted her parents to like me and i figured I could achieve this by making them laugh. So halfway through dinner I lifted up my leg and blew out a mive fart. Then I started laughing hysterically and fanning my arms to waft the stench around the table. I expected them all to crack up laughing but they got offended and left! Was this fair of them? why were they so mad? and how would you have reacted?