Hadn’t really considered it, but there is a difference in PWP of fanfiction vs original fiction. In a PWP fanfic, you have the benefit of context, of characters that have already been built up, so if you write a Dumbledore/Riddle PWP, there’s already a backstory that the reader’s aware of. But, a PWP original fic, without a world already built up, then it’s likely more gratifying, IMO, to google and get more vivid imagery/video of said act (assuming the act is legal).
I’ve got trouble there, myself. In my current fic, I’ve got some scenes that are rather close to PWP, but those do serve a purpose, even if the main focus of the chapter’s on the act.
I’m a weirdo! And it’s fine that there’s a line you won’t personally cross – that’s the original question of the topic Me, I don’t see myself doing [mpreg] anytime soon, but I’ll write teen/teen. I do feel some ickiness if it’s adult/teen, and I’ll strike Thor’s hammer at any character pursuing adult/kid.