Reasons girls get called sluts:
No, this isn't any stupid joke about master keys and shytty locks. This is the real deal, prompted by this recent phenom of girls who are shocked (shocked!) by guys who judge girls over this alleged double-standard.
Anyways, a lot of the whining is to the effect of "why is there a double standard zomg" and other fairly standard timeless questions. As someone who pretty much is an expert only in relationships and sexual subjects, allow me to explain:
Men are easy. Men are always easy. It is not hard for a woman to have sex with 50 men in the span of 3 months. Hell, an average-looking woman could probably knock off three or four guys in a single night if she put her mind to it. All she'd have to do is find a bar full of desperate, drunk military guys.
By contrast, it's hard for men to have sex. Men would love to have sex with three women in one night, but it's hard to do. That's where the term "easy" comes from - a woman is "easy" when having sex with her is (relatively) not a difficult proposition. So, a guy who lays 50 women in 3 months is a "pimp", because he is doing something challenging. A woman who has sex with 50 men in 3 months isn't necessarily more sexually appealing, just more willing to have sex.
Now, the complaint that stems from this is a favorite of women today: that somehow 'society' has invented this system to keep women down, to oppress women, etc.
This is bullshyt. Some evolutionary psychology is helpful here: men want to impregnate as many women as possible. This mentality is evolutionarily desirable and thus has been selected for over millions of years, in the males of many many species. By contrast, human women can only be impregnated by one man at a time (and there are similar limits in other species). Therefore, she must be picky. After all, if she mates with just any old man, her offspring will be less likely to survive, and her genes will be less likely to be passed on. Thus, she picks the most fit male to mate with.
Let me repeat: For men, being promiscuous has been selected for evolutionarily. For women, being selective has been evolutionarily selected for.
But to top it off, men have evolved to prefer selective women. First, if a woman is having sex with many men, her pregnancy might not be his offspring. When he invests resources into feeding the child, he'd better be sure it's his. Second, if a woman is having sex indiscriminately, her female children will also be more likely to have sex indiscriminately, which will ultimately negatively affect his own genes' survival.
It gets even worse though! If one woman is having sex with all the men, she's monopolizing the gene pool -- she's decreasing the chance of other women having sex with fit men. And these other women want a shot too. So women themselves have evolved to dislike other women having sex with lots of men.
TL;DR: Men have evolved to be promiscuous. Women have evolved to be selective. Men have evolved to prefer women who are selective. Women have evolved to dislike women who are promiscuous.
Note - I am attaching no moral judgment to any of this. I have no problem with men who are chaste or women who are promiscuous. These are just the very deep-seated reasons why our biology compels us to think of men as pimps and women as sluts.
PS: This is based entirely on science and I'm not making some moralizing grand stand for the most part, but considering the above, imagine what you do to a guy if you run the risk of him finding out that this woman he's committed to has been so easily accomplished by so many in the past. If you're to be his "one" and his life partner, it's going to be extremely difficult to get over the biological impulse of jealousy and just outright hurt.
the truth will set these dudes free
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