She's just going to make him come? - Keltham thinks as he reaches the edge without any obvious attempts at delay. That's a bit surprising - maybe there's magic here for restoring male ejaculative potential as a special case of healing? Or maybe she can't read him any more than he can read her.
Keltham was hooked up to some complicated biofeedback machines, which another society might even have deemed embarrassing, in order to learn this next part; which is why he's guessing that the natives here won't have it.
Even so he has to shut his eyes and concentrate, as he clenches some muscles and relaxes others, and comes just a little into Carissa's mouth.