Fag stag and fruit fly are slang terms for a heterosexual man who either enjoys, prefers the company of (or simply has numerous friends who are), gay or bisexual people.
The latter term comes from the derogatory term for homosexuals (fruit).
The term, which originated in the United States in the 1990s, is the male equivalent of the more common slang - fag hag, a term which is part of hag-ism; the identification of a person with a group—usually united in terms of sexuality, gender identity, or shared sex—of which he or she is not a member.
The term can be used as a pejorative or as a term of endearment within LGBT communities.
Usage
The colloquialism is used, albeit rarely, primarily within the LGBT communities.
Mainstream shows, such as Queer Eye for the Straight Guy, Queer as Folk, Will & Grace and others, often explore the mainstreaming of friendships where differing sexualities play a complement rather than impediment to friendships and social situations.
In addition, homophobia has become less prevalent, and efforts to confront violence and hostility towards sexual and gender minorities, such as gay-straight alliance has helped lift some of the stigma attached to having LGBT people as close friends.
A 2002 episode of the MTV series Undressed was titled "Fag Stag" and centered on a gay wedding.
When Radar magazine debuted, its founder and editor-in-chief, Maer Roshan, called competitor Details' editor-in-chief Dan Peres, a "professional fag stag," stating, "Let's get one thing straight, Peres is not gay. But, his magazine sure seems to be."
An alternate use of the term fag stag is a male gay icon celebrity who is either straight or gay, and is loved by the gay community, and has "gay buddies," such as: Kevin Smith, Ben Affleck, Eric McCormack, Justin Timberlake, Colin Farrell, Matthew Broderick, Robbie Williams, Ben Cohen, Darren Criss, Nick Jonas or Daniel Radcliffe.