Twilight: adolescent hormones, independently wealthy, idle, far from any family or authority figures, has her own place for the first time in her life, bookish nerd with no social experience, constantly supervised prior to leaving home--y'all got it wrong. Twilight is the perfect storm of hyper-sexual late-adolescence, the kind they made R-rated teen movies about in the eighties. The moment she discovered that she was sexy enough for people to actually want to fuck her she would have embarked upon a no-holds-barred vaginal rampage. Pity Spike who has to witness it.
Fluttershy: the animals count, and she has lost count of how many of them she has AHEM cared for. Severe social anxiety would have made banging her own species more of a chore than a diversion, so she just doesn't do it. Other ponies would be more than willing, but she never picks up the signals and would likely balk even if she did.
Pinkie Pie: sure she parties, but the way she parties is like a five year-old. She does not get around as much as folks think. She was date raped at least once and was not particularly upset about it afterward. Everyone had fun, right? So it's all good.
Applejack: not as prolific a zoophile as Fluttershy, but when you work on a farm with animals you fuck the animals. Any farm girl who says she doesn't is a liar, and we're talking about Applejack. Unlike Fluttershy, Applejack does not have crippling anxiety keeping her from taking advantage of her athletic figure, so she has enjoyed a romance or two for a little while before becoming bored with it.
Rainbow Dash: hormones turbocharged with athleticism make getting laid a bi-weekly event which she would think of as perfectly normal. It's all just for fun, though. Being as full of herself as she is, she gets what she needs and moves on without hesitation. She has broken hearts and not even noticed.
Rarity: it's complicated. She lives on her own but in the same town as her parents and frequently entertains a pre-adolescent little sister who can be counted on to blab about every single last thing she sees. On top of that, being a hot chick has the unfortunate drawback that every less hot chick in town is eager to knock her down a peg. All that means that any affairs she has need to be kept strictly on the down-low. As such, she needs to be very discerning about her partners, to her perpetual frustration. She is hot enough to get laid at will, but she cannot afford to. The futile search for Mr. Right compounds her frustrations. As a result, she has taken to switch hitting with the justification that while boys are for marrying girls are for fun. It allows the agonizingly slow process of vetting the local stallions to continue without a miserable dry spell driving her absolutely bonkers. Unlike stallions, there are plenty of eligible mares about, and, hey, she is in the fashion industry where a little homosexuality is practically encouraged.