Sometimes the "for pay" part is just an industry thing where you'd watch gay porn that is still specifically all gay porn, but would have to follow the actor's personal lives to figure out if they're straight.
There is, however, that kink that comes with either getting a straight person excited about getting turned on to having gay sex. There's a "forbidden fruit" kind of mythos built around the idea that there was always something they wanted to try with another person of the same sex, but were afraid of asking or indulging in without the opportunity that a gay porn scenario would offer.
The stuff with gay people having straight sex, however, has a more sinister tone to it. While the idea of "converting" a straight person to the gay life is something people exploit in bullshit rhetoric, we live in a time where we can counter that with the idea of the fluidity of sexuality that isn't as hardwired as it used to be. The idea of "converting" a gay person straight has much more overt issues in that they often promote a creepy message of ways to "cure" gay people that people actually take into consideration.