Twilight: Straight, easiest of the bunch.
Rarity: Straight, I think so, at least. If not that, then she'd be Bi.
Rainbow: Lesbian, c'mon, you guys, I'm not trying at all to stereotype the "athletic one", but she gives me a strong lesbian vibe.
Applejack: Bisexual, this was quite tough, but I'm putting my foot down, here and saying she's Bi.
Pinkie Pie: I...I genuinely have no idea. I don't think SHE even knows!
Fluttershy: Asexual, she's either uneducated on the topic of the birds and the bees, or she just isn't attracted to any gender.
But hey, who am I to judge...