And let's not forget, the state already doesn't make the same demands for a lifelong relationship that the Church does, so heterosexual marriage under the state and nation already is not the same as the Catholic sacrament of marriage. It's really this simple: if you believe that the Church's teachings on marriage must be enforced upon all citizens with respect to homosexual couples, regardless of their personal faith or culture, then you should also be campaigning against civil divorce. You should also be campaigning against the state recognition of any kind of marriage that the Catholic Church does not recognize as falling within the faith—no atheists can marry, no Buddhists, no Muslims, no Jews, and frankly I think a lot of the various Protestant / so-called Christian denominations are on shaky grounds as well.
Unfortunately I don't have time to get into this further. But I'm going to post what I have for now.