Re: "Muslims are well-integrated in Britain – but no one seems to believe it"
I guess for us 'religious types' marriage is seen as closely connected with having sex (and raising children).
What other reasons would there be for civil unions? I assume legal ones? Or just wanting to make a formal commitment to each other?
Note how I split "Civil Union" and "Marriage" apart above. Civil Union to me is solely for the legal benefits, which are numerous. Everything from tax implications, to visitation rights and surrogate decision making, to estate issues, legal commitment to one another (requiring legal divorce to dissolve), etc, with legal paperwork put through city hall. Marriage to me is the spiritual or traditional or emotional union, with a ceremony in a place of worship (or wherever), which has no legal attachments and can be recognized or not recognized by others as they see fit.
If you look at the history of marriage you will see that they started off being very similar to my idea of civil unions, though the arrangements were a bit off by our standards today. Women, who were considered property or considered the more vulnerable sex in need of protection etc, were given to the husband by the father, usually a dowry was involved, and the husband would protect her etc. If you look at gender roles within marriage in Islam in fact, you see duties that are owed to one another between the partners and rules for them regarding those outside the marriage. These are things that can be included within civil unions to make them binding in law, like any other contract. I personally find the traditional marriage contract from ages past as anti-woman, but modern civil unions need not be that way.
If the state wants to get involved to make these "rules" of marriage mean anything then it has to be a formal contract, and if we are to keep the church and state separate then it has to be separated from the religious or spiritual version of "marriage". I don't see why you can't have both and just keep them apart. You can have your civil union and your marriage. I can have my civil union without a marriage. The homosexuals can have their marriage too, and you can go on telling them it isn't a "real" marriage according to Islam and they can shrug that off since they are not muslim.
For those people you mentioned who should not have sexual intercourse for medical reasons (such as closely related relatives), do you think that should be determined in the union vows? Or a promise made to that effect?
I don't think civil unions should have "vows". I think civil unions should have written and binding legal contracts. I think incest should probably be discouraged outside the contract, by the state. Maybe a law should apply, or maybe just an educational campaign. I am not an expert regarding the affects of incest and inbreeding, so I don't know which is more appropriate.