It's fairly obvious that whatever sex lives the Mane Six have had have been not very successful ones, given that they don't have mates. Now, one may argue that they could have mates and the Show simply doesn't mention it -- but that's rather a large part of somepony's life to omit.
Not that the Show would depict them having sex -- simply that said mates would be occasionally hanging out with them or talking to them, unless thiese are the coldest damn sexual relationships I've ever seen -- and that would be massively OOC for any or all of them.
There's also the minor point that the Mane Six are Embdiments of Virtues: and tend to score high even in the virtues of which they are not specifically-embodiment. Casual promiscuity totally does not fit those sorts of characters. That would be true even if Equestria were at least as promiscuous as our own culture.
Since I had to make assumptions about them to write about them, I went with the following being true about them in YOH 1500, when they rescued Luna.
Twilight Sparkle (17, Virgin): The most virginal of all six, since at this point in her life she hasn't even been romanticlly kissed (obvious from her later reactions to Humanoid Flash Sentry). Original reason for assuming her a virgin was her friendlessness -- one is unlikely to acquire lovers if one can't even make friends (yes, I know about the exceptions to that rule, but those generally also require feelings of worthlessness, and Twilight Sparkle is confident from Day One).
Rarity Belle (18, Non-Virgin): In one sense the least virginal of all six, since she's the only one with an actual extensive dating history (and the only one to whom the term "dating" could be correctly applied). Original reason for assuming her experienced was a certain brittleness in her manner, and tendency toward despair, which looks a lot like something broke her in the past. Also, Rarity when we first meet her is not what one would call realistic about love, and given her beauty that could have hurt her badly when she was younger and more naive. She was seduced at 12-13 and pregnant at 13-14, losing the foal to a miscarriage; she's dated since but deliberately avoided actually having sex. She eventually (starting around 1504) falls in love with Spike the Dragon; they become actual lovers at some point, and marry in 1508.
Rainbow Dash (18, Virgin): Yes, I know; she's boisterous and in the uncensored version probably bawdy as well. But if you look carefully at her reactions to events, she gets both excited and embarrassed in anything resembling a romantic or sexual situation (you can especially see this in her early interactions with Soarin). Dashie is very obviously, orgiinally, very much pretending to a sophistication she doesn't possess. In the SWSV she loses her virginity at 20 to Fluttershy in YOH 1502; after that love affair fails in 1503 (due to the shocking revelation about Fluttershy at Cadance's wedding), she is somewhat sexually-active but not promiscuous (she isn't comfortable with any love which does not involve mutual Loyalty).
Fluttershy (20, Non-Virgin): When I read Bad Horse's Fluttershy's Night Out I had my explanation for why Fluttershy is so horribly afraid of other Ponies at the start of the Show. Thus, she lost her virginity, to "Nosey," in YOH 1496, when she was 16 years old. After this she doesn't have sex again until she becomes Rainbow Dash's lover in 1502-03; after that she becomes increasingly sexually-active; eventually becoming Discord's primary mate in an open relationship which produces offspring. She is naturally polyamorous, and the most promiscuous of the Mane Six (though that's not saying much). Her polyamorousness is a Changeling Royal trait, though by actual Changeling standards she is very sexually-restrained. But then she was raised Pegasus.
Applejack (22, Non-Virgin): Applejack requires some thought. On the one hoof, she's admirable, beautiful, warm and extremely likeable. On the other hoof, she's incredibly honorable, even more so than Twilight Sparkle, so no way does she do casual sex. And she's very obviously single, and displays no romantic interest in anypony.
The obvious conclusion (to me) is that she is pining after somepony she loved very much and lost. Thus, I created the character of Landscape Carrot, who was her childhood best friend and teenaged sweetheart, became Applejack's first (and only) lover in 1498 when they were both 20, and then vanished. I was orginally going to have him be a rogue, unworthy of Applejack, but when I started thinking about my Changelings I realized it worked far better if he were a Captive, and one who develops unusual insights into the Changelings in consequence.
Applejack saves Landscape, and they marry, in YOH 1508. They remain married until his death from natural causes in the mid to late 16th century YOH.
Pinkie Pie (Virgin, 19): As I've explained in one essay, I think Pinkie has to be either chaste (the Pie family code of honor) or promiscuous (she's very hedonistic). I don't see her as being moderate in this regard: Pinkie desn't do moderation very well.
The reason I went with her being a virgin is that, in the first three seasons, she shows absolutely no romantic or sexual interest in anypony. Her concept of 'party' is explicitly (and scornfully) stated (by Chrysalis in Cadance-form) as being that of a little filly, and Pinkie Pie takes this as a COMPLIMENT. (To show where the writers' sympathies lie, the real Cadance herself has the opposite opinion -- she enjoys the childish aspects of Pinkie's party).
Pinkie's romantic feelings are shown awakening in Season Four (YOH 1504), when she's 23, which is fairly late for her to start reacting to stallions like a mare. My explanation is that Pinkie Pie is very neotenous, and in terms of romance a late bloomer, for all her extroversion.
The trigger was the events of "Magical Mystery Cure" (late YOH 1503) in which she accessed the memories of an alternate Pinkie who had already, at the same age, married Big Mac. This made her emotionally aware that she, personally, could fall in love and get married; that she was no longer just a filly.
It's notable that one of the two stallions toward whom Pinkie expresses some interest is Big Mac -- one of the things she does in YOH 1504 is become Big Mac's friend, and she is willing to alter the timestream to do this. In the SWSV she falls instead of Cheese Sandwich, but Mackie was definitely a possibility.
Pinkie marries Cheese in YOH 1507 under my current chronology, and first has sex (ever) on her wedding night. Afterward, she blesses Cheese by altering the timestream so that he leads a literally charmed life -- any future worldine in which he will be killed is inaccessible from their POV. They remain married into the Poniternity.
Those are my versions of the sexual histories of the Mane Six; they apply to the Shadow Wars Story Verse. They do not necessarily apply to other writers' storyverses. Still, they wer chosen based on what I've seen on the Show -- they are not simply random.