It is possible that he might not be human; if this is true then that means he is doing things that he is not used to doing. That would explain why he threw up, because his body was reacting to something strange
Maybe cuz it was his first time and he didn't understand why people would put their tongues in eachother's mouths and he really didn't like the sensation of silaiva? Idk i find it weird too
Maybe the saliva was humanising him and his body was rejecting it or it was extremely painful that he threw up
Okay I still wanna know why he threw up