What i think:
Twilight I think she is bisexual. I don't say this because her colors looks like the from the bi pride flag, no. We have seen her having a crush on Flash sentry, and i think she have given a bit romantic to mares and other female characters. Well... maiby i say that because i ship Twilight with Rainbow dash XD The romantic orentation is for me aromantic for Twi ;3
Rainbow I say she's a lesbian. Don't get me wrong, i hated it for a long time when ppl have sayed she's homosexual. But now... her romantic orentation is bi- or aromantic. (Not sure yet)
Fluttershy LESBIAN. Clearly, for me. I think Fluttershy don't likes stallions, for some reason. She more prefers mares. I think she only likes stallions when they are kind, shy and love animals, like her. When you ask for the romantic orentation, i would say panromantic (Prefers mares though)
Pinkie Pan. Over and done.
Rarity I see Rarity as an straight ally. Though, she is biromantic, and maiby would date Applejack. But i mostly seen her as hetero. Why? Because she clearly felt attraction to Blueblood and other. I've seen her blushing some times to Applejack, so i'll say biromantic.
Applejack I think... sorry, but i really think she's at least heteroflexible for some reason. I know she wants to "build" an strong apple family, but i see her having much attraction towards females. For me, she looks like a pansexual aromantic hehe XD.
If you don't wanna read everything, here's the text without all the explanition XD
Twilight Bisexual aromantic
Rainbow Homosexual bi- or aromantic (Not sure yet)
Fluttershy Homosexual panromantic (Prefers mares)
Pinkie Pansexual panromantic
Rarity Heterosexual biromantic
Applejack Pansexual aromantic