Is it acceptable to call lesbians gay or is that term reserved for gay men?
Gay, in the Dictionary, is defined as: Of, relating to, or having a sexual orientation to persons of the same sex. So, yes, the word can be applied to both men and women.
Back in the olden days (the 1950's, say) homosexuals of both sexes preferred "gay" to "homosexual". As Women's Lib took hold, many lesbians began to feel that their concerns were not being met by the gay men's community, so they began to insist on the phrase "gay and lesbian." These days, it's common to see "LGBT" (lesbian, gay, bisexual and transsexual) in printed material. Even within the LGBT community, there is some chuckling about this proliferation of letters.