Against as marriage as an institution has been the bedrock in all societies for the creation and bringing up of children from time immemorial. Humans take longer than most animals to reach adulthood which is why this public commitment came about. If you allow it when their is no pro-creation element, then why not also allow same sex close relations to marry? There are also other legal difficulties like how do you define adultery and is it grounds for divorce?
I also think there will be many problems for the church and other people that are against it on religious grounds.
I'm pro civil partnerships as I think it made things more equal from a legal point of view and provided a form of commitment for those in same sex relationships.
At the grass roots level this has done much damage to the Conservative party where it wasn't in the manifesto and the party as a whole wasn't consulted. George saw it helped Obama's popularity and decided it would do the same for the Conservatives! Still the good news is that is has helped increase UKIP numbers.
Dithering Dave now has a tough decision to make, is he going to propose to Nick or George?