He said: "If I was gay, why wouldn't I admit it? It wouldn't harm me and my mother wouldn't freak out."
Cowell is also quoted as saying anyone who doubts that he is telling the truth is "mad".
Journalist and author Tom Bower, who interviewed numerous friends, acquaintances and business associates of Cowell for the book, said that he had found no evidence to support the idea that Cowell is gay.
He wrote: "The pertinent fact is that not a single man has offered any evidence of an advance by Cowell despite the promise of enormous financial reward."