As far as I know, most of religions view marriage as the beginning of a new family. Funrthermore, they view reproduction within a married couple as a needed and blessed aspect. Since they believe giving birth to children is one of the main reasons of marriage, they view gay marriages as abnormal (gays can't reproduce).
In my opinion, it is ok for gay couples to exists, but do these couples really need to get married? I mean, you can't possibly be a truly religious person if you are a homosexual (your religion itself is against your homosexuality, so you do!) and gay marriage may cause bureocratic issues, so can't gay people just be living together in their house without being married and be happy like that (just asking)?