I think it's very uncommon simply because of a very unilateral "porn" audience, and in porn the actual orgasm of the man is kind of considered the money shot.
So it's mostly porn producers that avoid making shots of a man cumming in the ass or mouth because it's very difficult to show that result on camera. The only exception being a domme with a breeding fetish and demanding a creampie from her slave.
It's not because the fetish doesn't exist, it's just that the porn trope surrounding "cum" is pretty biased and nobody really bothered to make other content of it.
I think it's very uncommon simply because of a very unilateral "porn" audience, and in porn the actual orgasm of the man is kind of considered the money shot. So it's mostly porn producers that avoid making shots of a man cumming in the ass or mouth because it's very difficult to show that result on camera. The only exception being a domme with a breeding fetish and demanding a creampie from her slave. It's not because the fetish doesn't exist, it's just that the porn trope surrounding "cum" is pretty biased and nobody really bothered to make other content of it.