This being a dream, he could bite and drink his fill and possibly it wouldn't even kill Jack. Possibly it wouldn't have any consequences at all, beyond potentially spoiling them for lesser couplings in the future.
But then, fantasies tend to do that anyway.
He gasps and bucks when he feels Spike's teeth press in, and then whines when Spike lifts up, and the grip on his cock falters too. He blinks a moment as he processes the comment, and then laughs. "Would you believe it's intentional?" he pants out. Okay, having to wait while he removes layers as an incentive to think for a moment before he engages in sex might be way down there on the list of reasons he wears this outfit, behind things like reminding him of the time he bet the Doctor, and making him look distinctive and damn gorgeous, but it is, actually, on the list.
The laughter clears his head a fraction, enough for him to grin up at Spike and say, "On the plus side, I am very good at getting out of it all, if I can just..." It turns into a grunt as he heaves and tips Spike off him, not so much to get on top but just to get out from under so he can sit up enough to shrug his coat off and reach down to unlace and kick off his boots.
As he does so, it seems like the room shifts around them and suddenly they're not in the warehouse any longer but in what could only be described as a love nest, the bed no longer a four poster but a very generously sized divan, the sheets now red, the room around them cosier and much more luxurious. Jack pauses a moment from removing his boots to takes it all in with an open mouth. "Huh."