It doesn't make sense to him but Carissa thinks it makes sense to her and maybe that should be enough.
Keltham can tell that he wants this.
Keltham can tell that he's afraid.
Without any sense of hesitation, Keltham looks within himself for further details, this being the obvious thing one is trained to do next in that cognitive situation.
He's afraid that he'll do this wrongly, if he tries to do it, and not give Carissa what she expects in return because there must be something even if she doesn't understand what it is, or just make a misstep that she isn't expecting, because he's an alien, and she won't forgive him, and Chelish Governance will decide that the alien is too dangerous to have this kind of sex with, and he'll never again get the thing he just discovered.
He's scared of betraying a thing that it means to be Keltham, that he does pay fairly what he owes.
He's scared that -