Its generally that way because its guys saying it. Guys understand guys. Males are designed to be horny animals wanting to fuck as many people as possible to spread their genes. That's the way we're designed. Females aren't designed to do the same thing, as they're designed to fuck once, get pregnant, and spit a baby out 9 months later. While that's a grossly simplified version of it, I'm sure you get my point.
Its pretty much the same way in regards to a father and his children, too. If he found out his son got laid at the age of 15, he'd be proud of him. But even hinting towards their daughter having sex at the same age and he'd set out on a crusade to kill the guy she had sex with. Sadly, sex is more taboo when it comes to females. Females that's had sex with 20 guys are considered a slut not because she's had sex with 20 guys, but because she's considered "easy". For some reason because the general population have the opinion that girls are supposed to be hard to get, and pure. Men on the other hand is generally accepted and widely known to, simply speaking, fuck as much as they can. Thus, a female acting male is considered a bad thing.
However, on the other hand, the same goes for guys, but completely opposite. For males, it is considered a bad thing to act in a female way. For example, and as mentioned before, he will often be ridiculed if he hadn't had sex before. Simply because that's what's expected of him, because he's a guy.
Besides, double morals goes with a lot of things. There's an old Chinese saying that goes "Kill one man and you're a murderer. Kill several and you're a monster. Kill a whole nation and you're a king, but kill the whole world and you're a god."
Either way, in a way, yes, it is double morals. But only because you base it on your own opinions and expectations of the sexes.
It's actually very complicated, when you look at the roles we fill as genders in the family archetype, or even the dating archetype. Females generally are the ones who consent to sex or not, while males are generally the ones who consent to commitment. This is because women are the ones who bear pregnancy and all the troubles that come forth, and males have traditionally been responsible as providers for their families. What this means today? I don't know. People can use contraceptives to the point that sex is generally a no risk of pregnancy proposal, and women can provide for themselves and children, as well fathers can be legally indebted for child support.
Of course not all women are capable of doing that, some men are deadbeats and sometimes contraception(or the people using it) fails. In these cases, yes - a woman's sexual barriers and a man's responsible both come into question.
Then within the context of a solidified relationship or marriage, there's the possibility to cheat. As a guy, I generally assume if a girl is hard to get into bed while dating her, then she is unlikely to cheat on me. This is simply because... she's demonstrated the ability to hold off. Some people would say being very easy is a sign of low moral standards, but I don't think morality really comes into question with 2 unattached people. In that regard, a very easy girl might be extremely loyal in a relationship and never cheat, while a harder to sleep with girl might occasionally cheat if she really likes the other guy and he's been working her for a while. But from a guy's perspective, I'm going to bet on the hard to get girl, from personal experience, and I bet statistical likelihood.
You can say the same for the males, we may be more likely to cheat if we had high sexual partners. Though, here's an interesting perspective.
Back to evolution, men are more fearful of women cheating on them - the possibility they can become pregnant with another man's child, and the guy will raise that child, is a fear.
Women are more afraid the guy will leave them for another woman, as their livelihood is dependent on the man.
Therefore a guy might be more concerned with how many guys his girlfriend slept with before him. A woman on the other hand might be more concerned with what the past relationships were like. How many times did he fall in love?
This has statistical implications to the modern, real world - in how our society views different actions and holds different genders to different standards. I'm not saying on any intellectual level it means anything, in that cheating is cheating, hooking up with someone you just met, is the same for both genders. On a moral level, it's the same.
How we actually look at prospective mates on the other hand... quite different. I'm sure if I asked all my male friends, they'd say, "famous whore? Sure I'd bang her. Date her? No fucking way!" It's just the reality of it, and by a woman becoming promiscuous, she makes herself less desirable as a person to actually date. The same as a man, by being flat broke is less desirable as a date.
