Why does that sound better than doing a normal harem with girls?
Seriously, that makes it even better with the writer actually developing the characters more with the plot than making an OC whose OP and has Issei/Shirou Protag Eroge CHarm and luck EX with the females falling for them instantly.
Which is sad because that means the writers, especially if they do self-inserts, are probably horny and indulging on their personal fantasies with their waifu characters not caring abut their own personalities and want to bang them.
I don't mind what people ship with who. Whether it be from Fate or other works of Nasu, Symphogear, DxD, Fairy Tail, RWBY, Neptunia, Senran Kagura, I honestly could care as long as they try to not deviate that much of their character or degrade them into peeps that just want to get into the MC/reader/inserts pants.
Granted it is a fanfic so a canon characters traits and personality will be bound to deviate to an extent, but those kinds of stories Ultra brought up like with having Saber in a harem or something just twists my gears there.