I agree that it would be nice if laws reflected moral facts, but unfortunately people disagree on what moral facts there are and whether religion is any basis for moral opinions. In particular I found religion through the study of morality, and continue to hold that moral opinion comes before faith. My opinion of course.
Now, the fact that we each hold opinions on the relation of law to morality gives us no reason whatsoever to conclude that they can or cannot be divorced from each other. What does seem to infer a divorce between law and morality is the fact that we hold different opinions and need to reach a political consensus about how we are to deal with those differences. Where the law is decided by political consensus and not anyone's particular morality, then it seems that morality has no necessary connection to law.
Certainly we can say that in reaching that consensus we will probably need to share our moral opinions in order to figure out how to accommodate them, but this is not to say that these laws are based on moral sentiments or facts. I take it to be a tautological fact that where something is accommodated, it is not foundational. The basis of law is instead political.
The thing is that some people disagree with you that a gay partnership does not "really fit the definition of marriage". Personally I don't have a problem with calling same-sex marriage a civil union in legal terms and calling it marriage everywhere else, some people think that if civil unions are marriage in all but name, you might as well call a spade (civil union) a shovel (marriage) legally as well. It certainly saves making two separate [sets of] laws.
What isn't acceptable about this? It's legal recognition, rather than social. You don't have to acknowledge them to be married in the eyes of your church.
If gays are asking for legal recognition for a relationship that your religion doesn't sanction, then it seems that they're asking for a political recognition and not a religious recognition. After all, if they shared your religion then they wouldn't get married. But they don't follow your religion and do want to get married according to theirs. Denying them the right to exercise their own religion as they see fit because your religion doesn't like the way their religion works seems odd, as well as contrary to the political rights that you desire for the exercise of your own religion.