Some writers want to write Equestria as a sexually-promiscuous culture, one at least as promiscous as our own. Sometimes, they assume that because Equestria isn't Christian, the Equestrians would have pretty much no sexual morals whatsoever. (In doing so, of course, they pay Christianity an inadvertent and probably undeserved compliment -- they wrongly imagine that non-Christian societies are sexually amoral, which was precisely the mistake of the bigoted 19th-century Christian missionaries in dealing with native cultures in Polynesia and Africa).
This is possible in part because the Show only indirectly depicts sexual morals (though what it indirectly depicts implies a culture with reasonably high moral standards). Because the characters never explicitly discuss the subject, it's possible to imagine them very sexually-active (which suits the purpose of porn).
However, anyone who has watched the first two Equestria Girls runs into a problem with this theory, which is that Twilight -- while amost certainly a full adult in her culture -- reacts to Flash Sentry not like an experienced woman considering merely the latest in her long series of Pony and the City sexual adventures, each no more meaningful than the last -- but rather, and very obviously, like a rather innocent young woman who has little if any sexual experience. (This may also explain some of the Flash-hatred, too).
I've noticed that those who want to envision Equestria as the culture of Brave New World have come up with a solution for this. Plainly Twilight is so focused on academic studies that she has no time to engage in the mad whirl of orgy-porgy that all her friends and everyone else around her participates in as a matter of course.
In the typical "Poor Pitiful Naive Little Twilight Sparkle" story, Twilight Sparkle has her face shoved in the fact that some characters (often friends or relatives) are having sex out of wedlock. Twilight is terribly shocked by this and runs to her other friends, only to learn that everypony is having Lots and Lots of Sex, including not merely non-marital but even loveless sex, and that Twilight Sparkle is foolish to think that life can or should work in any other manner.
Usually the way that this is resolved is that Twilgiht comes to realize the absurdity of her former sexual morals, and also starts having casual or semi-casual sex, which makes her much happier. Now that she knows about sex, you see, she can never be nor would she want to be chaste again.
The implied assumption here is that nopony really exercises sexual self-restraint out of morality, but only due to ignorance. When their foolish outmoded assumptions confront reality, they must be discarded, along with their morals.
The main theoretical problem with this assumption is that pretty much every Human civilization (and the Ponies are strongly K-selective childbearers and raisers like Humans, so this applies to the Ponies as well) has or has had stricter sexual morals than the proposed system of universal casual sex (and, if your embodiments of Virtues practice casual sex, it's safe to say everypony else does as well). This is a problem because, when a custom appears again and again in disparate civilizations, it is a safe bet that it has major cultural-evolutionary value, meaning that its absence would be what requires explanation.
The main evidentiary problem with this assumption is that in-Show we see a culture that appears to regard sexuality as romantic, special, exclusive, and generally linked in some way to either courtship for marriage, or marriage itself. Mr. Cake is horrified at the implication that his wife may have cheated on him. Flurry Heart is conceived and born after Cadance and Shining are married. Rarity is jealous of Trenderhoof's attraction to Applejack, the obvious casual-sex solution of the sharing him is not suggested by anypony. Spike is jealous of Rarity's attraction for Blueblood and Trenderhoof. Rarity is jealous of the possibility of Spike's attraction to Ember.
These aren't as obvious as Twilight Sparkle's getting all flustered at the most minor innocent physical contact with Flash Sentry, but what they tell us is that -- while Twilight may be less romantically-experienced than her friends -- her friends aren't just having sex willy-nilly either. Some may have some sexual experience, some may be virgins, but all of them view sex as something special which springs from love, not as just "relieving tensions."
Furthermore, it's very obvious that Twilight Sparkle could have lots and lots of sex any time she wanted, if simple sex was what she was aiming for. She's a single Princess and national heroine. All of her friends are single national heroines. Any of them could bring home a different stallion every night, if that was what she WANTED TO DO.
The complete lack of even circumstantial evidence in the Show that any such thing is happening (and the Show writers are masters of getting things Under the Radar) strongly implies that this is not what any of them want to do. Instead, what we see is circumstantial evidence that some of them are too prudish (Twilight, Applejack,), concerned with reputation (Rarity), or just plain inexperienced and afraid of getting hurt -- no matter how much they put up a bawdy front (Rainbow Dash) to behave promiscuously. In two cases (Fluttershy and Pinkie) they are so weird that most stallions probably don't have enough emotional points of contact with them to appeal to them.
But let's get back to the original question. Why is Twilight Sparkle so sexually-inexperienced? Is it out of naive and foolish ignorance, or out of informed and wise caution?
And here we get to the main reason why I don't believe in Poor Naive Little Twilight Sparkle.
She grew up at the Royal Court of Equestria, under the direct tutelage of Princess Celestia, who is a centuries-old super-intelligent being.
Let's pick this apart.
What do you think of when you think Equestrian Royal Court? Yes, pomp and pageantry, luxury and wealth.
But also: the center of political power for a continent-spanning empire -- the Realm of Equestria. A place where leaders representing major national factions jockey for influence over the Autocrat. There is a lot at stake. They would lie and cheat and steal -- do whatever they believe they can get away with - to improve their positions.
Celestia is morally good. Very morally good. But she's not at all "innocent," or "naive," or anything of that sort. If she was, they'd eat her alive, despite all her magical might. They would do this by seducing her to this or that cause with various emotional lures. Some of these lures would be romantic or sexual. Celestia knows all about such lures -- and why it's important to avoid them, why one can't assume that somepony who claims to love you really loves you, rather than that million-bit trade monopoly you're about to dispense.
And what is true of Celestia is true -- to a lesser degree, because Twilight is very young while Celestia is very old -- of Twilight Sparkle as well. Not only would Twilight have observed cut-throat politics and all manners of seduction and treachery, Celestia would have directed her attention to them, to prevent Twilight from being vulnerable to such strategems. She would have done this all the more because she loves Twilight, and would not want Twilight to suffer use and betrayal.
Twilight Sparkle is morally good. Very morally good. But she's not an innocent, though in terms of personal sexual experience she's probably the most virginal of the Mane Six. She can't be, or she would have drowned in the stormy seas of the Royal Court.
What Twilight Sparkle may well be is Oblivious To Love. And that may in part be a defense mechanism, a pretense to avoid seduction by those who would make political use of her without offending and making enemies of them. That's the sort of skill Celestia would have mastered many centuries ago, and would be glad to teach Twilight.
This lady, who was neither naive nor promiscuous
could tell you a few things about that.
In conclusion, I do not believe in Poor Pitiful Naive Little Twilight. Instead I believe in a powerful, intelligent and sophisticated Princess, who has high moral standards.