Because, the Church believes in truth. Because gay marriage is impossible, just like a square circle is impossible, the government’s claim that it is, is a lie. It is the Church’s duty to oppose governments which lie to their people, and to preserve the truth.
Also, it’s not true that civil acceptance of gay marriage will not force it on the Church. Already, the Church has been forced to close orphanages and other organizations because they refused to allow gay couples to adopt. So, in some ways, it is forced on the Church.