Mainly because I haven’t bothered to do any research, I’m not very clear on what it is that is being discussed/argued about.
There is nothing to stop a homosexual couple from getting their friends together and having a party, registering themselves as a civil partnership, and thinking of themselves- and describing themselves- as married. For what it's worth, I would think of such a couple in the same way they think of and describe themselves, even without a registration or ceremony, as I do with other couples. (I don't know any gay couples who described themselves as married, but if I did I would follow their lead. This might be moral laziness, but I am certain I would think of them as married first, and only then begin to won
I thought of myself as married, and was often referred to, and referred to myself, in those terms, long before I underwent an expensive and (to me) meaningless ceremony. You are as married as you think you are.
Presumably there is some difference in the legal relationship between the parties. In which case there are better ways of dealing with the problem. The law could do something so profoundly at odds with its normal instincts as buggering off and allowing people to do what they want, but that isn’t likely to happen. But given that marriage means many different things to different people, it would be a lot simpler to let people decide when they are married and register the fact (for tax/inheritance/child guardianship reasons it is useful to have clarity as to who is married and who isn’t).
Society/the state has no busy saying what marriage is, it is a hangover from the old days when other people thought they had to give you permission to do things (not a lot has changed).
There are different ways of being married. There are many people who could not imagine marriage to have meaning unless it was performed, or at least confirmed, according to the rites of their religion of choice. There are some who think of marriage as something permanent, and considered themselves as married even after their other half has had a judge say that they arenoy any more, or even after he/she has married someone else. The fact that two people can consider themselves married to the same person suggests that there is no single understanding of what marriage means, and no obvious reason why any one authority should have possession of the term.
There are people who do not consider others married unless they have inspected the circumstances and found them in accordance with their own views and principles. Claiming that your understanding of marriage is the only one that counts is quite impractical, since most other people won't care and will just ignore you.
So coming back to the original point, I don't understand why some gay couples are so keen to submit voluntarily to more than they have to. But since some undoubtedly do so wish, is it simply that they demand that other people treat their unions with the same seriousness which they treat their own, which seems doomed to failure, or is there in fact some important legal distinction between marriage and civil union, meaning that they are missing out on something?