A note on the legal system
Around the world - but especially in the UK - 'political correctness' combined with a growing litigation culture means that there are a lot of taboo subjects, and harassment or preferential treatment are dealt with rather harshly. Not hiring or refusing to serve somebody because of the colour of their skin, religion, sexuality, age, and lots of other factors is frowned upon from the legal system.
Should, for example, a man be chosen for a job over a woman because of the latter's lack of a penis, for example, work tribunals (not to mention the press) would have an absolute field day.
A note on weddings
In principle, weddings are quite simple. One human is fond of another human, feels that this fondness will never ever go away, and decides to create an ever-lasting bond as a symbol of this fondness. In addition, these two humans are given some legal, financial, and practical perks because they have gone through a ceremony to shout their fondness from the roof-tops, with a statistical high likelyhood of a very alcohol-fuelled party to seal the occasion.
The institution of marriage pre-dates recorded history, which means that it has been around for over 6,000 years. It appears that only relatively recently (in the past 60-70 years or so) people have come up with the notion that two people of the same sex decide to get married - which isn't to say that men haven't been rogering men, and women haven't been boinking women - but the matter is complicated by marriages being historically religious concepts. Religions, not generally being known for their libertarian view on life, subsequently have largely taken a dim view on same-sex marriages.
Bringing the two together
Now, we've established that racism, religionism and sexism are intolerated in many situations of everyday life - but it seems that sexism is okay in one particular instance: In same-sex marriages.
Say that we have a lady named Isabelle, and a fellow named Tim. Her parents are from from Nigeria, and has a deep, dark chocolate skin tone, and loves Tim. His parents are from Ireland, he's as white as they come, and loves Isabelle. If there was a law which prevented the two of them from getting married because Isabelle is black and Tim is white, there would be riots, and the word 'racism' would be at the forefront of everyone's mind - after all, they're both human, why shouldn't they be allowed to get married? They love each other for god's sake, let them!
Most right-thinking individuals would agree with the above paragraph. However, let's look at it this way: If Tim wanted, he could marry any woman above the age of 18 in London. Nay, in England. Hell, the world. If someone fell in love with him, and they felt it'd be the right thing to do, they could get married. There might be problems with, say, visas, immigration, and all sorts of headaches, but nobody would be able to prevent the actual wedding to go ahead, and the marriage to be an official, legal marriage.
As of November 2008, the world's population is estimated to be about 6.72 billion. Take off 15% who would be aged under 18, and we're left with 5.38 billion people in the world, of which 2.69 billion are female. That means that Tim can legally get married to 2.69 billion different people. On the flipside, there are 2.69 billion people which tim is not allowed to get married to. Why? Because they share his gender.
In any other situation, if there was a proposed law which was applied differently to males and to females, there would be an uproar.
"Men can drive 80mph in a national speed limit, while, women can drive 55mph". "Men are not are allowed to buy lightbulbs". "Men aren't allowed to publish books". "Women are not allowed to operate a deep-frying pan". All these made-up laws sound absurd, and yet the law in many places draws a line between men and women, and adopts a difference in the law:
Many would argue that any law starting with "Men aren't allowed to..." or "Women aren't allowed to..." are per definition sexist - which means that a law which effectively says "Men aren't allowed to marry men" and "Women aren't allowed to marry women" are just as sexist as "Men are allowed to drive 20 mph faster on a motorway" or "Women are forbidden from going out in the streets after 10pm".
How did we end up in a society where it's completely taboo to be sexist in some situations, yet perfectly fine to be so in others?
Why it's all absurd anyway...
Sex change operations, while controversial, are perfectly legal in many countries that at the same time ban same-sex marriages. In addition, many of these same countries have systems in place which means that you can be legally recognised as your new gender, in name, in passport, and in most formal documentation except birth certificates (which, in a way, makes sense, because a birth certificate is a documentation of who you were when you were born, not who you grew into when you grew up).
... Which means that you can be male, meet another male, fall in love, have a sex-change-operation, and then get married. Or you can be male, meet a female, get married, and then have a sex-change operation, which would mean you're now the same sex, but still married...