There's a difference between casual oral sex with a stranger and crossing that threshold with your live-in not-son in terms of significance. I can easily see her being comfortable with the first and not the other; or at least not yet.She allowed herself to be eaten out by some rando waiter in a restaurant the first (and only?) time they met, during the MC's birthday celebration. She's not exactly "difficult" or "chaste", so it honestly could happen at any point the author wants it to. Especially now that the MC is a little more all around confident.
I feel it might be more of a technical problem (cause making renders take time) than a writing one that requires the right elements and circumstances to allow for the event to happen
There's also a high probability that Parker's connection to MC, his parents, or his powers is more than it seems at first glance, and a possibility that some factors not yet revealed there would prevent her from being comfortable with it.
As far as a technical limitation, there's very likely nothing that would make Parker significantly more difficult to render than any other LI. If there is a limitation, it'd just be overall pacing, both from not being able to rending all 15k+ renders for a whole story in a single update and from a storytelling standpoint.