Father in law making me crazy

My entire family is OBSESSED with finding out the gender of the baby. I would totally be fine waiting until the baby is born, but hubby wants to know so we are finding out at our 20 week scan. My FIL is convinced he can tell the gender in advance with "the pencil test" and that it's never been wrong. Well he tried it yesterday and it didn't work because he was swaying all over the place. He wants a boy so sub consciously he was swaying to make the pencil show a boy. We were laughing and joking that it doesn't work and it's 50/50 of being right anyway. He got so upset and stormed off. Was grumpy for the rest of the day and wants us to come back over as soon as possible so he can try again... I don't understand why it's a big deal and he is taking an old wives tale so seriously.... 
Sorry for the long post, just needed to vent a little. Anyone else having similar issues with their families or in laws?