Let me just state my position, I have no issues with gay marriage, but there are already gay groups in the UK planning to sue churches for refusing to marry them despite promises from the UK government that this would not happen when they introduced the legislation. We will wait and see what happens.
If it passes it will cause legal nightmares down the line. As an example in a heterosexual marriage a wife has some predefined rights that a husband does not posses in instances of abuse and battery or rape. How will things like this be defined between a homosexual gay/lesbian couple? Will their be a legal requirement to protect one member of the marriage over the other? How are two husbands or wives going to be defined in law? If they adopt children who has presidency of custody if they divorce? How will it affect alimony law?