The comment came the same day that the Socialist government adopted a draft law to authorise homosexual marriage and adoption, despite fierce opposition from the Roman Catholic Church and the right-wing opposition.
Dassault, who is boss of the major defence group of the same name, owner of Le Figaro newspaper, and a right-wing Senator, said that if gay marriage was allowed, "there will be no renewal of the population".
"We'll have a land of homos. And then in 10 years there will be no-one left. It's stupid," the 87-year-old told France Culture radio.
He said homosexuality was one of the causes of decadence in ancient Greece.
"Total decadence, of course. It's the end of the family, it's the end of the children's development, it's the end of education. It's a danger for the entire nation," said Dassault.
The draft law on gay marriage, a cornerstone of the election manifesto that brought President François Hollande to power in May, has come under fierce attack in a country that is officially secular but predominantly Catholic.
It now has to be voted through parliament before becoming law. Hollande has promised it will be on the statute books by mid-2013.