In the porn industry, gay-for-pay refers to actors that identify as straight, but appear in gay porn. There are three explanations for this phenomenon:
1. In straight porn, male actors are paid much less than their female counterparts. Male actors in gay porn are, on average, paid more. Also, male actors in gay porn are much more likely to become stars than male actors in straight porn, where they're overshadowed by the female actors. For these two reasons, there's an incentive for straight actors to do gay porn. Criticism from gay actors has led some gay-for-pay actors to claim that they're bisexual to legitimatize their gay porn careers.
2. In gay porn, story lines that feature supposedly straight men being seduced into sex by gay men have become extremely popular. There is the allure of the unattainable, and power associated with being the first to sleep with a straight-identified man. For this reason, actors who market themselves as straight (regardless of their orientation) are sought after, and are a valuable commodity to agents and producers.
3. Gay-for-pay actors are actually closeted or bisexual, and the porn industry provides them a platform to experiment with their sexuality, or a stepping stone to coming out, under the guise of a career.
The following two clips are of a gay-for-pay actor named Aaron James. The first clip is from his appearance on the Tyra Banks Show (I can't believe I'm linking to the Tyra Banks Show - there goes what credibility I might have had). The second clip is from a promo for a porn site (and for Aaron).
More clips from the Tyra Banks Show episode on gay-for-pay here: part 1 part 3 part 4 part 5
And yes, Tyra's 'interview' style is embarrassingly bad. There's some interesting commentary in the comments section from an article on this episode here, at Queerty.
So which theory (or theories) seems most credible?
I imagine that all three are somewhat valid, although given the specificity of male sexual response, the third theory probably accounts for more cases. But, without data, it's impossible to say for sure.