To answer your question, it's about equality in a legal sense. A lot of people consider a legal marriage to be the same thing as a religious marriage. Due to separation of church and state, this is not the case. People who are both religious and non religious are able to get "legally" married in the U.S. Homosexuals would like to be able to be treated equally in the eyes of the law. A "legal partnership" may come with the same rights, but since it comes with a different label, it basically becomes the same thing as "separate but equal." The idea is not to legislate whether or not a religious institution can or must perform homosexual marriages, for that would also be a violation of church and state. It is simply, again, to get the same legal status as heterosexual married couples with all the same rights and privileges that come with that.