Gay-For-Pay is the slang term accorded to male (and sometimes but less frequently female) pornographic actors or prostitutes who identify as heterosexual but perform homosexual acts professionally. It is a very common practice in the pornographic industry, though the legitimacy of claim is heavily questioned as many believe that any person voluntarily and regularly engaging in acts of sex with people of the same gender are homosexual (or bisexual) by definition.
The commonality of this phenomenon has increased with the heavy fetishization in gay pornographic films of scenarios in which a heterosexual male is "seduced" into having a homosexual encounter. There is an overwhelming array of amateur gay pornographic movies that center around alleged "street hustlers" or young heterosexual men who have been misled into thinking they are auditioning for something more innocent (like an underwear modeling gig) only to be "tricked" into disrobing and masturbating or submitting to a homosexual act.
A popular example of this phenomenon is the Latino Fan Club [1], a film production company which specializes in producing amateur gay pornographic films in which pornographic actors who are billed as "Straight Latino Street Thugs" appear to reluctantly have sex with one another (but are almost certainally actually bisexual or gay.) The video production company website also offers contact information for hiring these "straight" models as prostitutes.
It should be noted that many people who have been involved in the gay pornographic industry admit that some gay-porn actors who identify as straight (i.e., as gay-for-pay) are not in fact truly heterosexual. Because many gay men consider heterosexual men to be objects of fantasy, the gay porn industry has, to increase sales, almost certainly overstated the number of heterosexual men who star in its films. Moreover, many gay or bisexual men who star in gay porn films may wish to be identified publically as heterosexual for a variety of personal or professional reasons.