For me, I think straight men in porn act as though they are fucking for their own pleasure and they look like they are having a great time. In gay porn, it often comes across as if they are fucking to pleasure the bottom. Even worse is when a gay top has to pull out and jerk off for a couple of minutes to cum because they can't get there otherwise.
As a bottom myself, I would rather see the top enjoying themselves than concentrating on making the bottom cum.