This has already been done to death and there was an easy answer given for this. I don't support this double standard at all, but it does make sense.
Women DON'T have to work to have ***. Now what do I mean? How hard is it for a normal looking girl to go to a bar, have a few drinks and start hitting up a guy or two and just start making out with him? Do you think he'll have a problem with it? No. Not at all, unless it's a very special case. It's the opposite case with men. Men HAVE to work to have ***. Have you ever heard of a fat ugly casanova? No, of course not. Men have to be well furbished, and have to know their way around a conversation. If a man does what I described in the first situation, it would be considered sexual assault (unless it was the odd case where the woman wanted that). Women don't understand truly how much power they have simply because of having a vagina. It's easy for you to get into bed, so that's why women who do it are frowned upon. As opposed to men, it means you can woo a bevy of women in some manner. That isn't hard to do for a woman because the average man with average morals is easy to prey on and or seduce. Biologically, this proves true too. With women sleeping around and taking charge, it's actually counter-intuitive to the evolutionary process. Men are the ones who seek out their mate to pass on their genetics (and hers as well). That's why chivalry nowadays is dead with the ever changing society. We've evolved psychologically to the point of pure independence so we don't rely on just the men searching out for women. But that creates a huge ripple effect and many double standards like you just referenced here.
OT: It's just a sad double standard that will be hard to combat. Sad but true. The woman would be immoral for considering it rape, but the man can do nothing about it in this judicial system. Although in the case of rape, if it does go to trial, there would be obvious signs of forcible interactions (like pinning her wrists or holding her throat to incapacitate her), so that could be used against her.