Spike was not by nature submissive with someone he'd only just met (and John might look like him, but he very definitely wasn't him) so there was a long moment of tensing and almost-struggling at the surprisingly strong hand on the back of his neck.
Eventually, though, he stopped almost-fighting because, fuck, John was a good kisser. Spike began to kiss back, not quite biting into the kiss because he couldn't - John hadn't given him direct permission to hurt him - but definitely giving back as good as he got.
When John broke off to breathe, he sat back, admiring the view. Once John had recovered enough to pay him proper attention, Spike fixed him with a hot gaze. "Give me permission to hurt you, pet? To bite you?" He licked his lips, tasting John on them - like himself, but not - imagining tasting him properly.