The downside is that it results in overall diminished social stability and cohesion. Marriage helps families by providing heavy incentives for partners to settle their differences and stay coupled rather than get frustrated and split. It provides a foundation of stability for and an obligation to children. It protects a dependent spouse (say, a mother who chooses to stay home and care for her children instead of producing her own income) from abandonment by supplying the dependent with legal recourse for support even if the breadwinner decides to blow it. It creates a default set of conditions for emergencies, death, etc., that would otherwise have to be defined contractually. It creates a social pressure for young couples to settle down and start having children.
I understand much of this is diminishing in the face of no-fault divorce and concerted efforts to make alternate lifestyles appear glamorous or beneficial (when in reality they are filled with despair, confusion, and turmoil), but that is the promise and benefit of legal marriage.
Simply deleting the term "marriage" and making everything a "civil union" for the sake of the feelings of religious people is ultimately vain. The same net negative effects occur from the equivocation of permanent heterosexual coupling and permanent homosexual coupling. Or did you mean we should do away with any default legal benefit and do all marriage-type relationships individually through contracts, etc.?
>You've got it backwards.
See https://en.wikipedia.org/wiki/Marriage#History_of_marriage_b..., where the first line contains "the institution of marriage pre-dates reliable recorded history", and then expands on implementations in "ancient Israel", "Europe", and "China". See also "Marriage in Ancient Egypt": http://www.kingtutshop.com/freeinfo/Marrage-in-Ancient-Egypt... , etc.