Why Women are Better
Women not only have very low levels of testosterone, but they have an extra special hormone that promotes good behavior: Oxytocin. A multitasker, this hormone is released during sex, pregnancy, breast feeding, and whenever a person shows trust or goodwill. Oxytocin has been linked to many pro-social behaviors in many experiments (detailed in the book), mostly empathy and trust. Women are more empathetic, and empathy is the basis of all moral systems.
Why Men are Needed
The obvious answer is that men are needed to make babies (although with the advent of science that's probably not true anymore). But more interestingly, testosterone has a nice side-effect: justice. Ironically, testosterone-filled men are needed to keep society in check, to judge and to punish wrongdoers. Natural selection allowed testosterone to hang on for this very reason. We are the enforcers and punishers of a functioning society (and the risk takers). Women, pumped with oxytocin, are too damn nice to punish people. It's important to know that men do have oxyticin in smaller amounts, but the problem with that: testosterone actually cancels out oxyticin. So when testosterone levels are high, we actually enjoy punishing people for their transgressions, rather than cringing. Who shall throw the first stone?, said Jesus. Crack her fucking skull! shouted some dick.
The good news, of course, is that we can and do transcend these biological limitations. Too much testosterone must be kept in check, and the same goes with oxytocin (too much can lead to too much trust). It's about balance. With knowledge, critical self-reflection, and love we can become better. Love can be learned, and it comes naturally for most of us.