It was pretty clear when John stopped trying to find mental purchase and make sense of what was going on, because he stopped just holding onto Ben and pulled him in.
Stopped 'just' returning the kiss and deepened it, groaned into it. Something almost 'catching' inside him, sliding into place and being just, absolutely, perfectly right. Taking Ben's intensity and giving it back to him with single minded focus. Not violent, no, but John at least sure as hell was not being careful.