I can think of multiple reasons a straight man could get aroused by seeing a dick.

First, erections don't occur only because of arousal, they can happen from adrenaline as well. I guess if you're a homophobe and are about to watch gay porn as part of research, you might get a bit of adrenaline.

Another reason I can think of is that most straight men see a dick when they watch porn, meaning their brain may make the association of "dick on screen = some hot nude lady is gonna show up".