"
By definition a “Marriage” is a union between a man and a woman. It is thus not available to same-sex couples."
As I tried to point out earlier, the definition of marriage is not a fixed entity that has not changed over time. What people mean when they say 'marriage' or it's equivalent in any society you care to mention has changed massively over time. There seems to be a great deal of sympathy in the UK (and I'm afraid in the long run it seems like it's only going to get stronger) for the idea that it is simply changing again.
Bear in mind that right up until 50-60 years ago roughly, if you asked people what a marriage was they'd probably say something along the lines of it being a legal/religious endorsement of 'a relationship' - relationships until recently only being conceived of as possible between men and women, so the 'man and woman' thing was an assumption rather than a hard and fast rule. Nowadays we know that relationships do not just occur between men and women. It thus makes perfect sense, to me, to alter our definition of marriage accordingly.