First of all, this nation was founded on the principal of "Majority Rules." As such, laws prohibiting certain behaivours can be inacted, if the majority of the people agree with it. Since the vaste majority (something like 80-84%) of the United States is Christian and apposed to Abortion (since you use it as an example) it is perfectly legal to outlaw this.

The real problem is that people want to engage in every kind of act without consequences. So, when other people try to pass laws that would make them deal with the consequences, they get angry.