This is now the 100th time this question has been asked. At least. I'll answer the same way I do everytime-It depends on the culture you put yourself in. If you choose to be with a group of people who will allow a double standard like that, then you have no one to blame but yourself. Some people hold to that standard, some don't. It's pretty much that simple.

cultural educational problem, it shouldn't be that way...

I for one don't believe that to be true.

It's a stupid double standard that's unfair and sucks. But you wanna know why the double standard exists? First, there's a reason behind it, and some societies have exaggerated it, while others have repressed it. The history, the gender equality, government support, and the ratio of males to females in your society affects how strong this stereotype is. But the biological reason behind this is rather simple. Women always know their children are their own, so women don't need to worry about infidelity for the same reasons men would. Since there's usually some chance that a man could be deceived as to his parentage, men care greatly about fidelity in a long term partner. Women care too, but men care more about sexual promiscutity. This bias grew into the ugly practice of devaluing women with more sexual partners. But it's not as if all males lose respect for women once they have had sex. Men also appreciate sexual prowess, some men are turned on by jealousy. Women becoming sluts easier than men is an old stereotype, but it's not the whole truth and it doesn't need to exist anyway. As for why it's good for a guy, it's just math. Males can have a ridiculous number of children in theory, given enough partners, time and energy. Of course, it can also be bad, because the girls that would make good girlfriends don't want a guy that messes around and leaves. So men have to balance their inner drive to mate alot with their inner drive to be attractive to the girls he wants to be with long term. Again, stereotypes often have a specific origin, but they are almost never the whole truth.

It's not good if guys do it. Do you realize that 25% of the population now has a sexually transmitted disease? If your guy has had sex with a lot of women there's a good chance that he has a surprise for you, and not a good one.

Who said it's good if guys do. In this day of serious STDs and AIDS, it's irresponsible and stupid, girls OR guys.