as i mentioned in the post preceding this one, marriage is a largely religious institution, but because it's not the exclusive domain of judeo-christian belief systems, it's unconstitutional to maintain the concept of a stricly heterosexual concept of marriage because it's effectively declaring one religion's practices to be deserving of legal enforcement over others.

anyways, marriage is used in specific in regards to a lot of matters where "civil union" isn't largely because "civil union" hasn't existed in any meaningful way as society developed.

i'd go through them, but honestly, there's people who personaly went through those differences a little further down the replies who i believe would be happy to elaborate for you and far more knowldgeable.