The problem with this argument is that it is akin to telling women not to fight back when someone is trying to rape them, since it's inevitable. If there are no moral absolutes, and the "progression" of societal change is inevitable (social Darwinism), then it is not only futile but wrong to try to stop change. In fact, might as well speed it up and bring on the pedophiles.
Actually, it is the duty of everyone who can distinguish right from wrong to defend what is right and fight what is wrong, not to cave in and accept rape as inevitable.