I think the only reason the homosexual movement wants same sex marriage is to destoy the institution of marriage. It is the only way they can get society's stamp of approval for the way they want to live. Once they get society's approval in the form of government action legalizing same-sex unions, whether they are called marriages or something else, then they will be able to use the cudgel of government coercion to force society to put homosexuality on a par with heterosexuality in every way.
It will only be a short time before public school teachers are mandated by law to teach children that there the difference between homosexuality and heterosexuality is just a very minior technical one and they can find fulfillment in either one.
Note the quotes I just posted.