This is for kindergarten

There are straight men who fap to other men fapping.

Is that gay?

While it may not make them "gay" per se, it sure doesn't make their case as a heterosexual any more convincing. The only reason I can think of which would possess one man to masturbate to the sight of another following suit, without it proving the former to be at least bisexual, is if he looked not upon the shaft, but the hand. Maybe the first man envies the dexterity and strength of the other; he watches and envisions the second man's hand in the place of his own. He's aroused by the motion he witnesses and not the penis itself.