The "double standard" that so many, so often, refer to with such knee jerk predictability is not truly "double" at all. It is actually the same standard, philosophically speaking, for both the man and the woman. It stems from the value society places on success and accomplishment.
You see, a man's sexual innocence has never been a measure of his virtue. Historically, just the opposite has proven true. "Be strong", "Be a MAN", or "I'll make a man out of him yet", or "are you a man or a mouse", or "make something of youself" these historical phrases all stem from the idea that when a baby boy is born, regardless of how adorable he may be at that time, he is expected to not just grow to become an adult. To be truly considered virtuous i.e. "worthy", he has to grow up and be "A MAN". He has to be accomplished. Otherwise, he is just a weak, aimless, slacker. He's not tough, not accomplished, not capable, not a leader. Instead he is thought of simply as kind of a wimpy guy, maybe a nice guy but he's never really put into the category of male archetype that society looks up to as the fearless, virile "WARRIOR". As such, to be a "REAL" man means to be accomplished along the traditional warrior archetype (regardless of how many hours you may actually just sit at a desk and keyboard eating Doritos each day). It is about conquest, setting goals and achieving them, and displaying untiring discipline in the face of any defeat encountered along this path. So, historically, except for a few short periods of time (Victorian age, the 1950s, etc) sexual innocence has been anathema to a man's virtue. To be a MAN meant to have made sexual conquest.
Now I am not suggesting that quantity in excess is a good thing as the "quality" of those conquests must also be considered. Picking up a street hooker is hardly a "conquest". There was no "pursuit", no effort, no "wooing", no "charming", no need to stick by her for any length of time to "figure out" the woman, or any of that ($30 and "hey baby, will you suck me? does not count toward wooing or charm!).
Basically, you could think of it as though when a baby boy is born, he is starting off with a score of "zero". As he accomplishes more and more in life through discipline and hard work, he racks up "points", if you will. With enough points (not just from sex but from everything he achieves/becomes) he eventually will be accepted by society (and himself) as "A MAN". Moreover, he too accepts society that abides by this archetypal social role. They reflect and complement each other. If one fails to a significant enough degree, the other is doomed to fail as well
Then there are little baby girls. Adorable, sweet, innocent. Wonderful little beings just like the boys but the difference is, they are, in many ways, considered perfect little creations. They are already at the peak of their success. They haven't yet become hardened by the cruelties of the world that boys are expected to run headlong into and conquer. They're just born and yet, they already have racked up a huge point tally. The little boys are still trying to accomplish belching and crapping and the girls have them beat hands down simply because they haven't yet tarnished their innocence. So long as they don't become like the boys or, worse yet, like the MEN, they remain "accomplished".
Sure, they can star on the track team, get great grades, go to college, get more great grades, become a brilliant doctor, write some books, etc, etc, etc. That's all fine and, if they were guys, it would "count" tons toward the "point tally". That's not to say it doesn't count toward the woman's tally because it does count...just not as much. You see for women to "get their points", and tus be considered virtuous and "worthy" they have to accomplish one great thing: hold on to that innocence with which they were born. That is their test of discipline. That is wherein their hard work lies. All the other stuff is difficult, but it doesn't test them to the same degree.
You see, it is no accomplishment at all for 90% of woman to go out to a bar, dance club, party, etc and find someone with whom to have sex. There is no "conquest" at all in that act. Basically, all she has to do is raise her hand. It is analagous to a guy picking up a hooker. Aside from some minimal effort to get in the car, drive to the destination, and make his or herself known, it was cheap, easy, and required only that the individual succumb to their desire. It is not in the least bit virtuous as it took absolutely no discipline. Why? Because all the guys are trying to make a "conquest".
This is not to say that prudishness = womanly virtue. On the contrary, women too are sexual beings, and with the ultimate MAN to whose conquest she succumbs, her sexual desires are free reign and explore. This recognition is precisely why it is such an accomplishment for a woman to remain sexually innocent. A grown man who is sexually naive isn't frequently considered "accomplished". In fact people might even start questioning "what kind of a dork is he that he can't 'get' a girlfriend".
On the other hand, in opposition to the feminist notion that being sexually liberal equates to "empowerment", isn't it a greater expression of self discipline and power to be a smart, accomplished, well read woman with classically valued characteristics of grace, presence, beauty, style and charm....and, on top of all that, the steadfast resolve that it would take to maintain sexual innocence in the face of so many attracive qualities? Qualities so sought after and valued both historically and to this day that other women would be humbled and men would be beating a path to her door in hopes of becoming the "victor" of such a worthy conquest?
There is no double standard. It is the same standard. The standard is that society has now and for centuries admired and revered hard work, accomplishment, and discipline. There is no discipline in a woman's promiscuity, just as there is no discipline in a man unable make any conquests in his life or who can only make cheap, easy, "guaranteed" conquests. As more and more women "empower" themselves via sex for sex's sake and the fun of it, they not only erode their own potential to become TRULY accomplished beings but, at the same time they erode the possibility for men to achieve truly significant conquests because the goal is so much easier to reach.
Of course their are many other acts of discipline along the path of one's life and to whittle it all down to sex is to greatly over simplify the matter. But, since that is what this forum is about, the other "stuff" is a bit outside the topic at hand.
For the boyfriends who have found themselves at odds with their love for their SO and the feeling that the rug just got yanked from beneath them, I would hazard it is because they have real depth to the significance they place on a woman's virtue and how it ties in with her degree of discipline. To discover that she has been with numerous other men and performed acts with them that in the boyfriend's heart he feels signified both her deep caring for him through her "sacrifice", and his accomplishing a profoundly important goal (that of having the woman he cares about bestow unto him a gesture of profound intimate significance) shatters his perceived foundation of the social ideal. Achieving that ideal is the ultimate conquest. It is almost like a life purpose. Not that getting laid by virgins is one's true life purpose as I'm sure some would immediately joke, but that within the minds of these aspiring "warriors" there exists somewhat of a template, a rule book, if you will. They got all suited up, studied the rules, and had those rules reinforced through their other male social counterparts. They have been exposed to the stories of "conquest" from those counterparts who gave the playback the next day after they "banged this hot chick" at a party last night. Then finally, after the boyfriend makes that really significant conquest and says, "Hey, she's the one, she is perfect for me and the epitome of the conquest I've been searching for", that's when he finds out that his conquest was nothing more than the equivalent of picking up the $30 hooker on the streetcorner. He realizes that this woman into whom he has invested so much emotional and ideological currency, is really no different than "the hot chick" banging guys at parties for fun on her quest for "life experiences". It erodes not only his good feelings fro her, but also the ideological foundation upon which he views society as being built.