Marriage originated as a social/political contract, and the idea of emotions was not present. At most, it was expected that you would grow to love the one you married, but marriage was decided based off of financial and political factors, not feelings. However, in modern western society, marriage has become predominantly based off of love, not off of other factors.
Therefore, now that marriage is love-centered, there comes a dilemma: why is marriage needed? If people love each-other, they do not need (though may want) a piece of paper to justify or codify their love. If people are only staying together because of a contract, they should not be staying together in the first place.
Marriage does not provide any benefits that are inherent to marriage itself. Various societal and financial benefits are the result of boons that could be granted without the institution of marriage. Better home environments and healthier environments for raising children are dependent on the people involved, not a contract.