Yes,marriage should be a religious institution.Whenever possible a marriage should be started in church.This way when troubles come along they have their spiritual life to look to.Earthly people and possessions may fail a person but God and Jesus will always be there for a person and help strengthen the bond of marriage.
Anyone who actually has studied a little history on marriage will find that marriage pre-dates all organized religions. (People were getting married and holding rituals to honor this act before mankind knew how to write!) In time, most religions decided to work with the idea of marriage and add their own flare to it. Which is fine, but that does not trump the origin of the idea.
Furthermore, there do exist religions that treat marriage as a completely secular act, such as Buddhism.
I believe religions are free to extend marriage in their particular way (e.G. Want to perform the ritual at an alter, or in a Church, in a Temple, or on some other sacred land, or under an olive branch, whatever) but the term "marriage" and all of the legal and secular concepts that come with it pre-date marriage and should therefore remain separate from religion.
I say this for tax purposes only. Partners should be legally bound together so that they get taxed properly and the system is not exploited. However, the law should not limit marriage. Personal religion should affect how individuals act, but the law should ignore everything when it comes to limiting gender and ethnic marriages.