It is part of an increasingly antique paradigm through which relations between men and women are framed by an asumption of male superiority -for example, in how many societies are the women expected to be virgins on their wedding night, but its ok if a man is experienced? The language follows suit: men are 'sowing their wild oats', 'doing what comes naturally' -but a woman doing the same thing is 'a scrubber', a 'tart', a 'slut' and so on. Societies have tended to create rules in times when life was nasty, poor, brutish and short -for survival. The morals we have inherited still determine a lot of behaviour -the Taliban in Afghanistan, for example, are known to arrest people who are not married if they are seen together, after which they can be flogged or even in some cases stoned to death -even if they are just teenagers. The equivalent reaction in the UK is to be 'Talibaned' by the Murdoch press which for years had made the assumption that marriage is some kind of sacred institution so that if a footballer, politician or film star has an illicit affair he/she is 'stoned in public' by headlines. The 'Slut walks' that began in Canada are a reaction to this 'moral hauteur' turning it on its head: the point being that women wear clothes of all kinds when they are going out and that doesn't automatically mean they can be labelled 'sluts'.
From an animal perspective, men are 'sluts' if by that it is meant their public behaviour is driven by lust -but we climbed out of the trees of that jungle a long time ago, treating people with respect doesn't take much effort, and most often brings with it greater rewards.