Majority: The majority of Americans report they know someone who is gay/lesbian.
Various studies and polls find that people who reporting knowing someone who is gay or lesbian are also more likely to support gay rights. A 2009 Gallup poll finds most Americans (58%) say they personally know someone who is gay or lesbian. Among self-identified liberals, that percentage shoots up to 71%.
It might be easy to conclude that knowing someone who is gay makes you more sympathetics to gay people and gay civil rights issues. On the other hand, perhaps the higher percentage of liberals -a group generally supportive of gay rights- reporting they know someone who is gay may well also express the reverse: gay Americans may be more likely to come out to heterosexuals known to be supportive of gay rights.
While such questions are interesting, they also leave a bit of a bad taste for me because they other gay people. Imagine how strange it would be to ask the same Gallup sample "Do you personally know someone who is heterosexual?"