Hmmmm, with my previous concern proven to be no longer worth worrying about by you guys, I've cleared my mind now. And as for how the scene can play out, I have some ideas in mind.
Asterion only learned about our(the MC's) affection(or love) for him through Nikos, so it's fine for his first approach of intimacy to us was only a passionate kissing and cuddling sequal. Maybe after giving him more time and space to gather his confidence and readiness, he'll eventually feel comfortable enough about having an intercourse with us(the MC). He may still be shy about it, given he's never experienced or enjoyed affection and love on an intimate and sensual level with anyone else ever since he's been given birth until now(not just the accompany like with his friends, his master or playing with the masters' kids). I think he'll eventually listen and give in to his cravings.