Even if Sam didn't purposefully drug Dean, he "took advantage" of him being non-consensually drugged by equally non-consensually stripping him, trussing him up, and gagging him while he was unconscious from drugs he had not agreed to take.

Yes, true, and it's not a problem that your lines as a reader are in a different place from mine.

This is why I say there are reasons to presume consent exists in fanfic unless it's otherwise indicated. I haven't spelled out in the story how in-depth the lovers' discussion of BDSM groundrules and what they could do to each other might have been, because it's a PWP and the story doesn't need that level of detail. But I think it's reasonable for the reader to assume Sam wouldn't have done such a thing were he not sure Dean would be okay with it. (Dean knowingly sampled the drug; he just doesn't remember that part.) And I only quoted a small extract above; in the story itself it's entirely from Dean's POV and I think it's pretty clear he's a consenting participant.

That said, you're right: the initial set up in the story is without consent. But nothing sexual happens before that consent is given. (Yes, you can argue that what Sam has done with Dean unconscious is sexual. In RL it would be; but I do believe fiction porn has different standards.)