Haha, you're right!
No further questions...William is obviously Mulder's son! Who knows how Mulder had been during his teenage years and lets not forget, Mulder is freaky special (more than William in a certain way). You can't get a girl in telling her greepy UFO/ Bigfoot stories all of time. Scully was an exception and who knows how she must 'performe' in order to control the Mulderesque virility of her partner