@Lovelyricepudding
*Well, except of course that for gay Christian couples, they'd see it as using it for the same purpose and that's the right that civil partnership didn't confer - the right to this specifically religious rite. Sure, the procreation part wouldn't work exactly by the book, but afaik Christians had never barred elderly couples or ones with clear medical reasons for infertility from marrying. So on one side 'nope, marriage only means a man and a woman' and on the other 'ah, continuing revelation, God doesn't really mind if we're same sex any more than he's bothered about mixed fabrics and prawns'. That sort of thing.
While I can certainly see why some gay people would takeoink'sview that CP was sufficient or preferable, it's a bit odd to talk about 'gay culture' as if all gay people had one specific culture. There would surely have been many living lives of quiet guilt or unwanted celibacy because despite CP they couldn't 'be married in the eyes of god'. (Im a atheist, I'm not saying this is rational but it's a comprehensible viewpoint.)*
But that is specifically a matter of faith. Are you suggesting we should legislate what people must believe? Or that specific beliefs are unacceptable? Lots of countries certainly try to do that. Just look at the blasphemy laws in Pakistan...
I think there are two elements when talking about marriage.
There is what you are talking about, how to allow for a variety of beliefs that may exist about marriage in a pluralistic society.
But there is also the question, if we have institutionalized marriage, what it is for. We can't write legislation about it if we don't know what function it's supposed to serve. There is really no need for a legal institution in order to recognize that people intend to have monogamous sex or that they are in love or anything like that, it could all be done in some kind of personal sphere.
We have limits on what we recognize as legitimate in terms of institutionalized marriage. Age limits for one, in the UK there are only two people in a marriage, you can't get married to a horse or car or sex doll. The former two have quite different expectations in some other times and places, and there are weirdos who have claimed to marry horses, cars and sex dolls.
Historically in the west reproductive role has been one of the most basic purposes of marriage, more so than romantic love by a fair way. So it was an entirely legitimate discussion to have, whether it made sense or was socially useful to have an institution that was predicated and revolved around issues of sexual reproduction, whether that purpose was in any way relevant in modern society, or if it was still useful, whether admitting same sex couples would undermine in some way the basic principle in the same way that saying women could include a person with the male reproductive role could end up undermining women's rights legislation.
That's all outside the issue of protecting religious belief. But what's important to realize is that people who would make those more conservative argument, like many Catholics, do so because their religion has that view about the nature of the institution. It's not, the law should say this because it's the Catholic view, it's I think the law should say this because it's an accurate way of thinking about society, reproduction, and marriage.
There isn't some neutral secular position we can base social institutions and laws on.