This belief was first identified by Sigmund Freud as an inability to integrate sex together with a loving relationship. While some people joked that it comes from Mommy Issues, Freud believed it to be the literal case. Specifically, by feeling alienated from his mother, the man would later on look for the woman of his life based on the love he never received from his mother. The man will look for the good and shun the bad aspects of women, and thanks to his upbringing, having a high sexual lifestyle is on the latter.
As a consequence, it ended up suggesting that only chaste girls have a chance at happiness; women who don't fit this mold are just trash and should be disposed of accordingly; some men use this as justification for rape and Honor-Related Abuse. It also neglects the sexual needs of the "good" women, and relegates those of the men to being fulfilled by illicit relationships. It could also involve creepy obsession, and Loving a Shadow, an idealistic fantasy, rather than a real person.
Many societies past, and some present, have bought into this dichotomy (see All Women Are Lustful and Defiled Forever and Honor-Related Abuse). The inexorable, albeit flawed, logic of the Complex holds that everyone can be categorised this way... which is bad for non-'Madonnas', as being a 'Whore' often means that others can basically treat you (as badly) as they like because, hey, 'Whores' are not moral and/or human. Plus, if you're a 'Whore', you chose to be that way, therefore whichever other people do to you is your own fault and your just reward, including death.
Additionally, the Complex teaches people that 'good/moral' females have no sexual desires, and that 'good/moral' males don't have sex with other people; just impure/immoral/sub-human people. Given that The Complex often goes hand-in-hand with monogamous heterosexual relationships, this often necessitates illicit/secret adultery for everyone to sate their sexual desires as it would be immoral for either spouse to actually want to have sex with the other because they are both 'too good' to indulge such sinful acts. Having sex with an immoral/subhuman person is absolutely fine, however, and is almost expected of males in these sex-less relationships because All Men Are Perverts. How Madonnas become mothers without the use of in-vitro fertilisationnote is politely glossed over... except when it isn't.
Commonly found in works before the 1960s or so, when people first began to publicly admit — in large numbers — just how silly the whole thing was. Remember, according to the Madonna-Whore Complex all women are either promiscuous, immoral seductresses or sweet, naïve ingenues and the elegant (chaste/sex-less) matriarchs they become. 'Good' people who are sexually active do not exist — or at least, 'Good' females. "Of course" a man will have sex with a few 'Whores' here and there. That doesn't make 'them' immoral of course, what a silly question, why did you even ask...?
Like many sexist double standards, this probably has its roots in the inherent uncertainty of paternity in cases where the mother has slept with multiple men. Before the advent of DNA tests, the only way for men to be certain that they had fathered children was to harshly punish women for sleeping with multiple men, in the hopes of making female promiscuity utterly unthinkable. Like many punitive measures, this eventually got out of hand, resulting in the Madonna-Whore Complex as we know it today.
