In the world where a non-hunting home-maker (ie. woman, as you posit) found a better hunter when the hunter she was with didn't do well, the hunters probably didn't feed the women when they stopped being useful. Hunters most certainly fucked everyone they possibly could, to maximize their gene transmission. Marriage prevents this. Hence the expectation that your SO sticks with you when the times are tough... Especially when the times are tough.

Even murders can be 'justified' by evolutionary biology but the court tends to put an end to further propagation of that gene :)