Trog said in rec.pets.dogs.behavior:

Anyway, I heard somewhere that if a dogs castrated, he
no longer gives off a male smell. So as far as an entire
male dog is concerned a castrated dog must be a female as
the castrated animal dont smell like a male. Would that
account for the old guy with his nuts gettin horny with the
pup ?

Paraphrased from Hitchhikers Guide to the Galaxy: "Almost
everything you know is wrong." This includes humping.

Nick, your old guy's response is just his way of showing who's
who in your house. Sometimes, though, it's just play, so use
your best judgement.

--
--Matt. Rocky's a Dog.