"...it needs to pay off in the form of story advancement and character development, otherwise it's just gratuitous, sensational, "edgy." And for the last several episodes, it's become clear that the only satisfying way for the mystery to end is for Rust or Marty to be the killer."
I wouldn't find it gratuitous at all - the whole idea the show is getting at is that Marty and Rust are bad men hunting worse men. There are no "monsters" in the world - darkness and evil aren't otherworldly, they're very real and very human. The terrible shit Marty and Rust have done only underscores that they're not out killing and assaulting women NOT because they're good, saintly men who aren't capable. They're both very capable of that kind of violence. And they certainly don't - at least not really - believe in any form of karmic justice that "punishes" evildoers. So what stops them? Choice. To straight up steal from Angel - "if nothing we do matters, then all that matters is what we do." So finding the people responsible and trying to bring them to some kind of earthly justice is what they've decided matters.
The question is: if the Yellow King acolytes are in EVERYTHING, will Marty and Rust be able to dole out any kind of satisfying justice? I'm not sure how far even vigilantism will take them, if the conspiracy is large enough in scope.
I wouldn't dare call True Detective "feminist," because the reality is that it does indulge in a lot of objectification - but I think it's equally clear that it is doing that in order to comment on objectification of women. I mean, it starts with the LITERAL objectification of a woman ("a paraphiliac love map") and all of the violent actions done by men against women are condemned by the show.
Ultimately, I'm most concerned that the show will wrap up with our heroes convinced that even small-scale human justice is impossible when you're dealing with people whose power enables them to abuse, rape and murder with impunity. But I hope not. I hope in some small way, the bad men make the worse men pay.
Parody or New York Times Vows piece?
As karion touched on a bit upthread, it's the idea that anal and oral sex are "less than" that bothers me, because that worldview essentially devalues gay sexual experience. Yes, we can assume that Scarlet means vaginal sex is more significant to her personally, but: she doesn't exist in a vacuum. Sure, she can define her sexuality the way she wants to, but the cultural assumptions that underlie WHY she considers p-in-v sex a defining experience are pretty fucking problematic.