No, just avert your eyes in the future
Yes, that's unacceptable
Watching porn at work is unacceptable, especially if it is freaking out his co-workers.
I'd hate to be the snitch though because the dude will very likely be summarily fired, hurting not only him but possibly his family as well.
An anonymous note at least gives the guy a chance to change his ways. If he continues after that, then he is just asking for it.
Hmm, what, no "yes" just because it would be soooo much fun to announce to the entire workplace in your Sunday best outdoor voice, why you suspect it is that ol' Billy Bob has this uncontrollable need to run to the toity so very often ?...and thank god too, 'cause you was so very worried he had a tape worm or somethin' really awful...see now, that ain't quite the same thang as bein' a big snitchy snitch doin' that snitchy snitchin' thang, now is it.