Here's my response to the government consultation on same-sex marriage:
The essence of marriage is that it is two different people committing themselves to be together for life. The experience of having to live with, and committing yourself to love, someone who is fundamentally different from yourself, is one of the key drivers for personal growth. It is within the context of two people committed to love the other "for better for worse" that children are best raised, because the couple have learned to accept each other. That is the best societal basis for tolerance.
The most fundamental distinction between people is gender, as recognised on passports and just about everywhere else. Marriage unites two people of opposite gender, who are thus very different and so as they learn to accept each other, so they learn to accept people who are fundamentally different from themselves. The same is not true of "same-sex marriage". It would be a union of two people who are the same at the fundamental level of gender (and of orientation). It therefore is a very different thing from marriage, and hence a different word should be used. It does not provide the same basis for transformation, or for growth in tolerance and acceptance of the other.