In my AU, the mane 6 are in a polyamorous relationship this will be my longest explanation on something by far, so here we go.

Overview:
They are all dating each other, and don't prefer one over the other. Their relationship started with just Rainbow Dash and Twilight. They both realized that they wanted a polyamorous lifestyle, and decided it would be best to let the rest of the girls know. Later that night, Rarity and Fluttershy confronted the two in request that they could join their relationship. Pinkie and AJ soon joined after some lovey-dovey confessions. The magic of their friendship only makes their love stronger, and it seems that their relationship was destiny. (Celestia totally knew)

Princess/Future Twilight:
No I didn't forget Twilight's wings here. Their relationship started months after moving to ponyville. When Twilight gained her alicorn wings, nothing in their relationship changed, she was just more busy with princess duties. My AU doesn't focus on future Twilight, but I'll explain that situation anyways. The rest of the mane 6 are dating still, but it's a little tricky for Twilight to see her girlfriends when she lives at Canterlot now, ruling over Equestria. They all still meet up once a month to hang out.