Firstly, I believe that while yes, "marriage" is a religious institution, the laws that marriage provides should not be. Legally, civil unions and marriages should be the same thing. Also, I think that, whatever a person believes religiously, all of the Judeo-Christian religions (Christianity, Judaism, Islam), at least, preach treating other people like human beings, and that God/Allah/Jehovah is the one that has the final judgement.
Stop legal marriage from being the same as religious marriage, and let two consenting adults who love each other be together. It's not like it hurts anyone.