This is strictly fanon. It's about my own universe, which I'm actually starting to consider calling "The Spoiled Barrel" universe. One Bad Apple spoils a barrel, but that's 'spoil' as in indulge and give good things. You could trope it as 'Spoiled Sweet.' Granted, I made certain i took great heaping gobs of personality from the originals, but by the nature of being ongoing and interacting, changes and revelations are inevitable.
Twilight Sparkle: Twilight is a genuine lesbian, in contrast to the general trend of bisexuality in Equestrian mares. She just barely dodged being a tragic shoujo-ai cliche by being too scholarly. While she was manipulated by pretty mares she was never really in love with them because her studies came first. And then there was Pinkie Pie. Pinkie Pie was everything she never knew she wanted in a mare. A mystery wrapped in an enigma stuffed in a flaky crust and colored frosting. She wants it to work, and wants it to work for the rest of her life. Among other reasons she abdicated, she was unwilling to face Celestia and Luna's grimly-accepted serial monogamy. She wants to live as long as Pinkie does, or at least be in that range.
Pinkie Pie: Modestly bisexual, leaning lesbian. Being a Cannonite (Basically Amish) that's not really how it is done. While they abandoned the Herding ways of the ancient ones, the all-earth-pony group does not really cotton to Filly Fooling, though they only do symbolic shunning if it happens. The Pie family is a modest abberation, they have not even symbolically shunned Pinkie. There are whispers of political expediency, but Goodman Pie is hardly concerned.
Applejack: Bisexual. She has always been open to all genders of suitors, mostly by the order of Granny Smith. She hated everyone brought to her, but loved the mare she found on her own. The contrasts are half the fun.
Rainbow Dash: Lesbian, with a glimmer of bisexuality. She's been mostly concerned with speed and skill, so had a Twilight-Sparkle-like detachment. That was half the problem with Gilda. The drinking and the bad attitude completed the deal. She is also an interspeciesist, at least in the abstract. Given her relationship with Gilda that is obvious.
Rarity: Bisexual. Her dreams were never limited by gender. Species, yes, but without malice. It was mostly because she knew how to be with ponies. She enjoys non-ponies are friends and has networked freely and well with them in Canterlot. Afte realizing that Pony Tales don't come true the way she thought, she came to realize that the simple expressions of love are truest.
Fluttershy: Complicated. In essence, she has single-target sexuality, like Tomoyo of "Cardcaptor Sakura" has with Sakura. Fluttershy was asexual and fairly aromantic until she met Rarity. Elegance, beautity, brains, care, generosity, all in one. Rarity was the only one she really could love. That she was allowed to is a dream come true.
Spike: Heterosexual, formerly self-identified single-target sexuality. The first elegant pony he met was Rarity, and his puppy love went on until he saw what was actually best for Rarity. After coming to realize there were more fish in the sea he embraced his uniqueness and started trying to get attention from other ladies. He's a strong interspeciesist, with preferences for pony-shaped women though he mayy or may not have a rising appreciation for Diamond Dogs, also-gem-eating creatures.
Scootaloo: Reluctant lesbian. 'Reluctant' in that mush makes her gag. Except with Sweetie.
Sweetie Belle: Bisexual but very lesbian-leaning. She isn't shy about mushing it up with Scoots.
Applebloom: Bisexual. Much like her sister she was being groomed for suitors of all genders until she got engaged to Zecora. Is now in a "Jail Bait Wait" situation until her sixteenth birthday, which is her scheduled wedding day. The old green crone has that etched on a slab of stone.
Zecora: Bisexual. Her training as shamaness gave her a long view of life and love. She knew the moment she met the filly that she and Applebloom would become wives. She was not "attracted" to her body, but her potential. She saw what Applebloom would be, and knew she wished that for her life partner. This could be seen in a "Cardcaptor Sakura" context as well, with Terada-sensei and Rika. Zecora has it easier: despite inadvertantly using old slurs, Granny Smith loves the buck out of her, and was the one that subtly (for Granny Smith) got her to "put a ring on it" so she could have a "zap-zap no takebacks" on her.
Big Mac: Straight. Or object-sexual. He has not been sexually active with Smarty-Pants. His love was platonic until she came to life. Then it was romantic but chaste.
Diamond Tiara and Silver Spoon: They are practically one pony in two bodies. Their (extremely) long and (extremely) happy marriage attests to that.
Snips and Snails: Homosexual, solely and exclusively. Had they the nerve to confess they would be the happiest little Colt-Cuddlers in Equestria.
Gilda: Straight-out lesbian. That's a giant no-no in the Griffin Kingdom, especially for a member of the Bald clan whose family is highly placed. Egg Grabbers are no longer subject to fines, jail time or "overlooked" assault but there is a strong social stigma. It's the reason she is an ex-pat.
Trixie: Lesbian. She can flirt and fool but she has passion only for women. She is also something of an interspeciesist, as Roa never turn away allies and those willing to help and care. Skill and willingness matter most, explaining her rather intense romance with Gilda.
Celestia, Luna, Cadence: Bisexual and serially-monogamous. I figured this out after thought. They will outlive any lover they take. But while they have them, they are the focus of all their divine love and care. One line, that I have not yet given to any of them would be, "I will outlive you, but I will never stop loving you."
Bad Apple: Straight... essentially. His sexuality was never that important, even when he had that short thing with Lysandra in Grain Gulch, or in any of the other implied times. Now that he has Luna there is no real need to think about sexuality: he is hers, she is his, that is all. But he can tease Colt Cuddlers like Rosy and Golden for his amusement and be very authentic about it.