Maybe, if they spend enough time together, Jack might eventually start to get that. And exactly why Spike was so pissed off about not being allowed to earn his soul on his own.
"Oh, now you're just giving me ideas," he purrs, because whether Spike's exaggerating or not, he's been in Spike's body and he knows just how keen his sense of smell is. So as well as leaving visual marks on Ianto, he could make sure to leave as much of his scent on him as possible too. Or fuck him in locations he might bring Spike to. Or something. He'll have to think on that one a bit.
And then, in an effort to prevent Spike from being right about sex being the only thing he ever thinks about, he adds wistfully, "None of that's even there any more. The place I spent over a century of my life, and it got blown up and filled in as if it never existed.