I don't think Ralsei would understand what an orgasm is at first, since his entire existence is to serve the Lightners, and recite the prophecy. His knowledge is optimized to fulfil fate, so self serving desires and instincts don't occur to him until later in his character arc.
Kris would probably show him how to masturbate in secret, and Ralsei would probably shut down from all the stimulation of his first orgasm.
Imagine him poofing into a pile of fluff from the feeling of Kris' soft tender hands stroking his shivering tender penis until completion.