Another stereotype is that gay people speak differently than straight people. That's often true, said Bailey.
"Gay men absolutely talk differently, on average. You can tell far better than chance who's a gay man from just listening to him say four sentences," Bailey said.
With Bailey's help, "20/20" ran a test in 2004 to see if people could tell who was gay and who was straight. Five gay and five straight men spent an hour mingling with dozens of people. Could the testers determine who was gay and who was straight?
People were accurate 60 percent of the time, which is better than chance. But there were plenty of wrong guesses, too, showing that the stereotypes can be way off. The man who most people thought was straight was actually gay.
Many people do think that gay men are more promiscuous than straight men, and in fact, Bailey said, gay men do have more sex partners. But, he said, it's because men, in general, want lots of partners and women, in general, do not. Women limit the amount of sex that straight men have.
"I think that the typical straight man would have as many sex partners as the typical gay man if he could," Bailey said.
But, we asked Bailey, isn't stereotyping harmful?
"Denying stereotypes means that people have to disbelieve what is right there in front of their eyes. That can't be a good thing," Bailey said. "Furthermore, there's nothing wrong with being a feminine man or a masculine woman."
But, of course, stereotypes can lead to ridicule and to violence. People have long mocked gay people, even attacked them, for being different. While there may be some differences between gays and straights, there are lots of similarities.