Why did Israelites kill every Midianite male and female except for the virgin females that they kept for themselves? Why is sexual immorality treated so harshly throghout the whole Bible, and especially harshly when it comes to a woman commiting it?

There is a percentage of men who make women sluts. Both these men and women are responsible for the destructive consequence of this behavior. The consequence of this behavior is the current state of the Western culture. Female promiscuity was never intended to be a standard but is treated like one in the depraved Western culture.

The worst thing a man can do is to contribute to it by making himself more attractive to sluts and/or generating more sluts. The next phase is likely be a transformation into what we see in the Latin American shitholes, where sexual immorality, sociopathy, violence, poverty, murders and all the stuff like that are rampant.