There is a longstanding convention among lesbians and gay men in the United States: Do not reveal the sexuality of a gay person to a heterosexual person; unless you are certain that the gay person does not regard his sexuality as a secret. Lie if necessary to protect her secret. Violating the convention by "outing" another person is widely considered a serious social sin.
David L. Chambers & Steven K. Homer,
Honesty, Privacy, And Shame: When Gay People Talk About Other Gay People to Nongay People,
Mich. J. Gender & L.
Available at: http://repository.law.umich.edu/mjgl/vol4/iss2/2