Religious marriage is all well and good but its civil marriage that counts and the reason is that marriage had that connotation before it ever got a religious one. The world over societies have rerwarded opposite sex marriage as it makes sense and serves as the best building block to better the society as a whole. Other relationships don't provide this and are not rewarded so.
The purpose of marriage has always been to regulate sex, but liberals and libertarians don’t understand this because neither wants their sex lives to be regulated.
Traditionally speaking human individuals did not choose their own mate. A person’s spouse has traditionally been chosen by their parents or other supervisory kin. You had to be lucky to end up with a spouse that you found physically attractive, and you had to be damned lucky to end up with a spouse with whom you could establish any kind of emotional bond. The choice of spouse was seldom left to the people who were actually getting married since marriage was meant to provide men with sex; to provide women with physical and financial security and to provide society with children with which it could perpetuate itself. Society dissolves (much as America’s society has dissolved over the past 60 years) when we allow men to seek sex outside of monogamous heterosexual marriage and when we allow women to seek security outside of monogamous heterosexual marriage and allow children to be produced and reared outside of the normal family structure that is provided by monogamous heterosexual marriage. The most religion can do is uphold what is already found in a stable society.