Tax break? Where? I pay more in taxes being married than when we were single and living together. Where is this tax break that you speak of?
I don't believe that you have to have a "religious" reason to have a monogamous relationship. I also don't believe that marriage itself is what binds you to a person. being "married" has not changed my commitment to my husband, it just changed my name. I'm committed to my husband because I only want to be with him, not because of some legal/religious/whatever bond that was made.
Why is there so much divorce? Because people are selfish animals. I'm not sure what will change that.