A psychological complex wherein a man (or lesbian, I suppose) is only able to regard women as a mother or a "whore", a good non-sexual woman or a desirable object of lust, but not both.
It's used in two contexts. The first refers to a man who regards a woman as beautiful and lovely until she has sex with him. Once she does, there's clearly something wrong with her morals (how could she give herself up that way?) or with her selection (i.e., he's got some serious self-esteem issues). Conversely, once their relationship is over, she suddenly becomes desirable for him again.
The second context is a man who becomes unable to be sexually involved with his wife once she becomes a mother. For one reason or another he now associates his wife with his own mother, the incest taboo kicks in, and he becomes unable to be intimate with her, pursuing other women or a mistress instead. The theories of Freudian psychology have a lot to say about the why and how of this.