David Cameron will note that Britain is one of the countries most in favour of gay marriage, with 66% in support. It's worth noting that most people in this country (58%) now have a work colleague, friend or relation who is lesbian, gay, bisexual or transgender (LGBT), a pattern which is by no means the case in all countries covered in the survey.
Sexual orientation is, I think, a complex thing and not reducible to the facile and outdated categories of nature or nurture. Likewise, rights can be complex. But the case here is simple, whether that way by chance or choice, gays and lesbians deserve the right to marry. Granting that right in the UK is a good choice, let's make it now.