We all knew why we shouldn't ship those two. There are many reasons, but I think other fans have talked about them enough. I just want to add one thing. Black Butler has so many other great ships! For example:
Sebastian x Undertaker (Who wouldn't want to ship the two most attractive characters?)
Grell x Madam Red (Both red, both wanted children but can't have them. The sad ending makes it such a great ship! Also, gay! Why not?)
William x Sebastian (I don't actually ship them. I only do when I need to write something dark, because they hate each other. However, it can be a good one if you're into something like this. Both of them are so serious and hardworking. Just go for it. It's better than "that one"!)
Sebastian x Grell (It doesn't matter if it's a one-shot, smut, or anything else; this ship is great! They are both hot and can be villains. Imagine them committing murder together or Grell helping him eat souls. I could talk about them until the day I die!)
Undertaker x Grell (This is the ship I focus on the most. I see their queerness (in both senses) as similar. Additionally, Undertaker's vibe is both mysterious and silly. I imagine him to be incredibly loyal.They can do each other's hair!)
William x Grell (This one is more canon. Yana once said that Grell actually loves William and not Sebastian. Also, it's nice to see William grow from an asshole and a transphobe to a kinder person. Or he can remain himself but still be better for Grell. Once, I read a fanfic where William was asexual, and it's so good to imagine! It even makes sense because William always denies Grell. Maybe he thinks it's for her own sake)
Undertaker x William (It seems a little odd, but since William (at least in the anime) is kind of a fanboy for Undertaker, why not? Undertaker will tease him, and he will be annoyed. It's fun)
Undertaker x William x Grell (f you want a poly ship, it could work for you. I think William is more suitable here than Sebastian because Sebastian is more likely to be sexually shippable. Romantic toward both people? He can't. But if that's what you want, then go for it.)
Claudia x Undertaker (This one is probably canon. We don't know, but it's so good, especially because of the sad ending. I even think that Undertaker wants to bring her back to life. And Claudia was the Queen's Watchdog. (Was she, or am I remembering a fanfic?) So, a brave woman and a Shinigami lover? Hell yeah!)
Of course, there are other ships too. But ships with Grell, Undertaker, Sebastian, and William in them, or at least the first three, are the best ships. When they are not human and exhibit villainy like Grell, it makes it even better. I don't know about you guys, but I love a ship with lots of blood and violence. Whether it's with each other or toward other people, it doesn't matter. And since we don't know anything about the past of these characters, there is a lot to create.
Also, something very important: I'm not saying that dark, sexual, or other themes aren't usable for the two mains (Sebby and Ciel). We all love something that hurts. Personally, I hate Ciel. I'll probably let Sebastian harm him in my next fanfic. Not shipping them doesn't mean you can't make them sexual. It's the freedom of a writer to do it. Just like people write about war and other ugly things. Why not rape? (I know it is bad in real life, but it also happens. By writing about it (as long as you don't present it in a normalizing way), you can educate others about it)Sebastian is a demon, so he would probably do that.
I also dislike the way some people say their relationship is like that of a father and son. What the heck?! Did you see Sebastian's memories? He hated that little bastard! Well, at least it's better than shipping them.
