It's like that's what both their subconsciouses are doing - making this, perhaps, into how it might have gone had there not been rules about who you could sleep with involved, making it natural. To Jack, it's actually pretty similar to quite a few dreams (or fantasies) he's had about Spike already and thus feels completely natural, except the part where they're in Sunnydale, because that came from Spike.
He squawks again as he's tipped onto his back, but it's only a momentary protest because, really, he's exactly where he wants to be. He laughs up at Spike and offers him the bottle of whisky that's somehow (this being a dream) managed to make it through the whole shenanigans without spilling too much.
"Well, if it's only the whisky you want, here you go!"