I agree with derkins that it is "built into our brains".
It's a scientific fact that women are weaker than men. It may be the reason why men find it in themselves to "protect" women from "danger."
But I blame religion and culture for this double standard. Why are women required to hide their bodies in Islamic countries? Why are men allowed to marry more than one woman in many cultures (Even in America. While it's illegal, some men have multiple wives)? Why are women expected to be virgins at the time of marriage? Why are 13-15 year old girls being forced to marry men twice their age? In some cultures, men get a slap on the wrist for rape but a woman is given lashings.
We humans must reprogram our minds. We must begin treating our women fairly. Humans are smarter than this.
I don't know about the rest of it, but the young girls married off to old guys is left over from ancient civilizations, when we were trying to produce as many offspring as possible. In most cultures, women were considered property, and some modern societies haven't grown out of those traditions. But I think as a species we should be able to move past that and give women the same treatment as men.
As for the "slut" thing, it's just one of those double standards we haven't grown out of. I guess because long ago men had many wives and nobody batted an eye.