Responses
I don't think he only JUST dry-humped him, though... I think he was masturbating him at the same time (and senpai came in his hand which is part of the reason he ran away in shame). Since he can't exactly masturbate chairs, maybe he wouldn't do any of that to those?
Wouldn't be surprised if he had some dog in his pedigree. Like those girls from that one anime. With his sudden dry humping of innocent bystanders. Who knows, maybe he does it to chairs too, in the privacy of his apartment maybe.
Wow, wait wait wait, maybe he did it to sempai's things too not just sempai, and some hypothetical chairs.