I would just like to say that content like this is incredibly offensive to basically the entire cast of Madoka. When it comes right down to it works like this are saying that all the emotions, feelings, development and relationships the cast have with each other are virtually meaningless in comparison to sex. And that they would abandon all of the traits I listed at the drop of a hat if it meant catering to some sexual act like having a threesome. Threesomes by definition are defined as a type casual sex and are considered something low brow because there are no deep feelings behind the sex, and if there are feelings they are cheap and not all that they could be if they were dedicating themselves wholly to a single person. Even if you personally see nothing wrong with threesomes by, again, definition they are a depraved act. The meaning behind this is that the cast of Madoka all have strongly defined characters who would never do something crass or depraved. So in content like this which implies the characters would do as such it degrades them from people who are true and faithful about their feelings and character to people who would trade real love for cheap love and sexual pleasure. This story does not even try to push the idea that feelings held between Mami, Madoka, and Sayaka are strong because as the end it implies Homura would join Madoka and Mami in a threesome as well, and they have no objection  to that. This the same as saying Homura, who spent years time traveling for the sake of Madoka, has cheap feelings for her and doesn't think enough of her to want to be true and faithful. And frankly, that's very insulting to say about her, as well as all the implications it has about Madoka, Mami and Sayaka. They have no true depth of character and are just there to have sex with each other because this story even indicates that none of them would have a problem have any type of sexual activity with the other girls thus meaning their sexuality is lose and their feelings of love are shallow which shows that the work has no respect for the characters or how they really feel and warps them to be depraved. There is no value in the love if they willing to spread it among everyone. It has no value because to love means to commit your whole person to someone, and you can't commit a whole to multiple things.

Now obviously this is just fanwork, but that is no excuse for being so bluntly offensive about the portrayal of the characters. While, as stated, this is not official content, it is still very insulting and degrading to Mami, Sayaka, Madoka, and Homura. Fanwork by nature is a parody of something thus the characters are a reflection of their counterpart in the official story. If the characters were not a reflection of their official counterparts than it would no longer be fanwork and would just be original content. This is why outside groups such as the fandom are able to insult the characters through the content they make or actions they take. Because these are reflections they are directly linked to their official characters, which is why if the fanwork is insulting, it is also insulting to the official character. It's similar to racism, while it has no direct harm it is still offensive and crude. This is especially true in a case like Madoka where the cast is well developed and all have strong feelings. So to imply that such strong feelings are actually worthless is rather rude to say. Sure you may think they are just fictional characters, but implying such things about them is crude, similar to making joking about a character who had a dramatic death.