Once you've Yensched, once you've had him in your head, once you've seen your lover in his clothing, how and where can you go from there? They have a mutual pact to protect each other, thanks to Aeryn and wormholes, so why all the drama now? The one thing John's never done is submit to him. He's had parts and parts of Scorpius in him; he's done the Chair and he's fought Harvey and called upon Harvey as a friend and killed him again, he's killed for Scorpius and been saved by him a million times; he's done his time in the hotbox. He's got Scorpius all over him, but he's never once asked for it. Never given in, because he's never had reason before. Now it's not just brain and hand and heart: it's blood. (But also this: the part of her that's hidden from him now, her child and her activities in the dark time, has been linked with Scorpius from the beginning. He's not just John's darkness now, he's hers too. The shadow and the anima, the hideous androgyny of Harvey's Lovely Daughter; and on the other side of the door is God.)
Scorpius grabs John's hand and cuts a finger. Blood begins to drip from John's finger -- Scorpius holds it up, opens his mouth, and squeezes a drop onto his tongue. He then wraps his tongue around John's finger and draws it into his mouth, sucking. Hard. John...watches. Scorpius finally pulls John's finger from his mouth, lowers it, and hisses, staring into John's eyes.
..Okay actually maybe it's not all that mysterious in this case.