This is just a suggestion but what if after the main character gets a taste of cock fucking both getting their cock to somebody else's and receiving one into themselves they can do it with other NPCs and maybe receive special benefits from it, like with the potion seller with him you could gain his ability to mix the potions that you receive into one mega potion effect, or with imp you gave me ability to grow somebody with your cum. For you to be able to gain the effects you have to do this more than one time with each one as a way to properly obtain it your body is slowly learning how to do it how to do what they do. Another detail about this is that once the player is big enough and their cock is big enough the Nyxs creatures rather than going through the mouth can go through the cocks entrance.