Why is it believed that anyone who is not straight is automatically promiscuous?

People who are convinced that gay is wrong and dirty will always look down upon any gay person even if it's obvious that person has not been ually active for years. And when they meet a straight person like a guy who is really ually active and promiscuous and even cheats on women frequently that person will often be hailed as a ual hero. It's sad really, I guess we have to keep our private lives private to avoid scrutiny.