It is a social contract for reproductive gender equality. In return, men are motivated to work more diligently at all these tasks. A corollary is a mutual contract among men to respect women and to protect them if necessary. Like all contracts, it is sometimes broken. But without it, men have less interest in respecting women, and more interest in exploiting them indiscriminately for sexual and genetic gains.
Rape is a crime against patriarchy as well as against women.
This is another reason why we find more rape, not less, when patriarchy breaks down. Either people are powerful and strong, or they're not. The reality is that both men and women have natural power in their own spheres, and use it both for and against one another. Obsession with the areas in which men are stronger denies women's own very considerable and superior power in other areas, and undermines cooperation with contention. The rape rate is highest in the ghetto, where patriarchy and civilization together have been allowed, even encouraged, to break down.