That's the number of Americans who self report as homosexual. It is substantially lower than the 10% gay rights groups have often argued. So, in a country of 300 million people, that would mean about 10 million people are gay or bi. In a recent Gallup poll, young people, who hold much more liberal beliefs on gay marriage, estimated 30% of the population was gay.
Numbers matter in this issue. It's easier to argue something is abberant behavior when it's found in a mere 3.5% of the population. Look for the gay rights movement to go after Gallup on this.