The anti-theism is in reaction to the terrible shit justified by religion.

Edit: posted early on accident.

Edit 2: I personally believe people's morals shouldn't require unprovable belief in the afterlife to work. Religious people often argue that religion is necessary for morality, but an agnostic approach is the only one people should live by.

If you sincerely believe you can be denied eternal life by going against God's will, keeping others from committing sin is a rational, moral thing to do. You're saving them by outlawing homosexuality or other similar shit. However, there's no evidence that God even exists, let alone what he wants you to do or that he can grant eternal life. Therfore, we should make sure our morality works without the existence of higher powers we have no way of detecting.

Belief in things that don't affect our material world can make behaviors that cause great harm seem rational. There are amazing people with strong faith, and terrible people with none at all. I just recognize that our common ethical principles cannot be determined by unfalsifiable ideas. Laws shouldn't be guided by religion.