I don’t know that there’s anything appropriate, I just go with cringe or try to make a stupid joke. Does anybody actually read porn comments? I’m not sure what else you can say? Serious critique of the lighting? Intellectual deconstruction of the porn’s intent? Cum-taste enhancement recipes? Comment on some feature in their bedroom/house/wherever?

We’re looking at pics of their nude bodies because it makes our genitals feel funny, the whole situation is just so absurdly ridiculous that nothing you say will be or not be appropriate. Though I guess complaining, insults, or implied violence is a general no-no.