Possibly a very simple reading of human sexuality, but if a man showed me photos of another man masturbating, I might suspect that it was, in fact, gay porn, or there was an assumption about my sexuality. I suspect this sort of material is rarely shared for any other reason.
The implication is that this is an image the presenter of the material finds appealing, and probably thinks I would also find it appealing.
Were it a woman showing another woman the same image, then I think we'd call it heterosexual porn, or at least erotica depending if it's in black and white.
