The sudden flood of desire that rushes through him then is strong enough that Keltham feels (falsely) like he is having trouble breathing.
Dath ilani male clothing is by default made to contain and direct erections in such fashion as to not be outwardly visible; if somebody is sitting on your lap, they may be able to tell. Yaisa can probably tell.
He cannot tell her yes, his brain is still confused and halted by that part of things, but such a thing as Yaisa has said, coming from a girl on his lap, has to be answered somehow.
"Carissa thought you were a masochist," Keltham says hoarsely. "Are you? If the answer is yes, I'm going to bite you." He's not sure if he's allowed to ask permission, but this seems like it should serve the same function.