But do you imagine inspecting the older guy's genitals as meaning watching him get an erection? Touching said adult erection or masturbating it? Observing the man feel sexual pleasure and excitement in the presence of the boy?

Yes - particularly since the boy in question is attracted to the man physically and sexually.

That seems like it might be much rarer.

I still don't understand what makes you figure this.

Especially if he starts out wanting to see an organ in action, but then observes sexual arousal directed at him and doesn't understand what it is.

And how do human beings learn?