Interesting theory on being able to document divorce rates to stem the tide of gay adoption. I'm not certain it will be a factor in reality. It could, and will, be argued that gay couples who adopt are more likely to stay together than couples that do not. We may also find that gay couples who are married stay together and have lower divorce rates.
I do not believe the availability of marriage will increase the rate of homosexuality in our society. If some sort of government recognition of the union of two people, increases committed, long term relationships, I think it is a good idea.
Your Church should not have to perform or recognize the union. There were, and I assume still are, Churches that do not recognize marriages if one parter is not of the faith. Same logic hear. I have no problem with our government, for the purpose of laws and benefits, only recognizing a civil union and Churches recognizing marrages.