It's what Clark means when he speaks about trust. He trusts Bruce knows what he's doing. In a way, he trusts Bruce better with all of this than he trusts himself.

When his mouth locks onto that small patch of skin, it's an excuse for Clark to close his eyes, immerse himself in simple things, like the build of slipperiness and friction between them, the slow build of pressure as he gets hard all over again, the tiny wet sounds from Bruce worrying at his skin, the slow rise of sharpness in the ache forming there. Clark is quiet as this happens, until Bruce lifts away with that last scrape of tooth, at which point he groans, complaint and relief and pleasure all at the same time.

—which is interrupted when Bruce presses his arms up and back and down. Starts new and louder when this latest bruise is pressed, when he says that, looks down at him. Clark has to wonder which part of this, which combination of things, seems to send him over the edge from interested into aroused.

He nods, breathes out 'yeah', a little like he was planning on avoiding talking altogether before deciding he needed to make sure there was no mistake, there.