Marriage, to me at least, implies Christian ties. Most Christians believe that homosexuality is wrong. Considering that during many/most/all? marriages in the U.S. have a reading from the Bible in them (i think? lol you always see them with a Bible, guess that might be wrong :- it does have some Christian ties. Civil Union has no implied or real connection to the church at all. So, you don't have to force Christians into (granted indirectly) supporting something they don't want to, while still allowing homosexuals the rights that they are entitled to.
Signed with the Sign that sells!