A lot of people believe in it actually, although I do not think it exists in the adult online industry. If there is any drop off of sales by sponsors it is because webmasters are working less. Factors like time of the year, weather, gas prices, politics, football season, tax day, etc... do not affect adult porn sales.
Mainstream, brick and mortar stores definetly, mainstream online (consumer goods) sure. But Porn, hell no. That is like saying people smoke or drink less because it is the summer.
The biggest arguement that people who believe in the summer slow down have is that kids are home from school during the summer. This one proves my point, the majority of the people who work from home work online. If they work with adult material they will have a hard time working while kids are home. 99% of the rest of the population are still at their office working, yes even during the summer when the kids are home. Just because the kids are home doesn't mean dad takes the summer off of work and stays home with them lol.

Please share your thoughts.