I understand the legal aspects of civil unions for gay couples, but I am only curious about the process? Is it a matter of just filing paperwork? Or do they go before an officiant to take vows as civil union partners? I realize of course, they may choose to have a ceremony with vows regardless of the legal standing of such ceremony.
As you guessed. no vows or ceremony are required, but then, they aren’t required by the legal aspects of a marriage either. Either one is just a contract - legally, it requires as much ceremony as signing a mortgage (in some countries, less). Most people entering either civil marriages or civil unions choose to add some ceremonial bombast to the event but this is because “otherwise it just doesn’t feel right”.
It varies from country to country and state to state. Some examples:
[ul]
[li]the procedures for a civil union in New Jersey are the same as those for a marriage - get a license from the city/town hall, wait 72 hours, have a ceremony before a clergyman, judge or municipal official.[/li][li]to form a civil partnership in the UK the partners must sign a document in the presence of two witnesses and a registrar; a ceremony with vows is allowed (as long as it’s not religious) but not required.[/li][li]to register a domestic partnership in California you just send a notarized form to the Secretary of State.[/li][/ul]
Incidentally, this also varies from one jurisdiction to another. Where I live, the law requires the officiant at a marriage to ask each party, “Do you, A.B., declare that as far as you know there is no lawful impediment to your proposed marriage with C.D. here present, and that you call all here present to witness that you take C.D. as your lawful spouse?” and each of them must answer in the affirmative, and then the officiant must say “I declare that A.B. and C.D. here present have been lawfully joined in a marriage.” Without those words it’s not a legal marriage (although an exception is made for bona fide errors or omissions).
But those aren’t vows, and they’re a lot simpler than what’s needed for many other contracts.
In France, a civil marriage is rather ceremonial. It must be performed by the mayor or one of his deputies and is usually going to take place in the town’s hall ceremonial hall. There might be music, in small villages there might be beverages served to whomever shows up. Guests and family will be present, pictures taken, the newlyweds will kiss, and so on.
Of course, the only part actually required is the mayor/deputy taking one minute to read whatever he’s required to read and having the two people and their witnesses signing the marriage form. Some people do marry this way, just showing up in everyday clothes with two witnesses, but it’s not the norm.
By contrast, a civil union (which doesn’t carry the same rights and obligations and can be dissolved at any moment by either partner) is only the two partners signing a form in some office. So, though in theory there’s not much difference (two vs four people signing a form in front of a deputy mayor vs a civil servant), in practice, it’s a completely different business.
For the record, while civil union was intended for homosexual couples, it became very popular, and 95% of them now are between heterosexual partners. So, it has become a “marriage light” or “pre-marriage” instead of an homosexual union. As a result, when homosexual marriage will be legal in some months, civil union will stay on the books as an alternative to marriage, an originally completely unexpected result. Such a situation obviously couldn’t happen in countries where a civil union has essentially the same consequences as a marriage, and is only different in name (such as the UK).
Too late to edit : I wanted to note that in France the only way to get legally married is to have a civil marriage (there are no marriage licenses granted to priests, pastors, etc…), so the ceremony at the town hall always take place.
(Most people also get married at their place of worship, usually later in the day, but this has no legal value)
[quote=“ctnguy, post:3, topic:655230”]
[ul][li]to form a civil partnership in the UK the partners must sign a document in the presence of two witnesses and a registrar; a ceremony with vows is allowed (as long as it’s not religious) but not required.[/ul][/li][/QUOTE]
(bolding mine)
Wait, does that mean that you’re not allowed to have a religious ceremony with vows if you want to form a civil partnership in the UK?
You can have a separate religious ceremony, and you can have vows at the official ceremony, but you can’t have any religious component at the official ceremony. (For marriage the official and religious ceremonies can be one and the same.) I believe this was going to be changed but has since been superseded by the same-sex marriage bill currently going through Parliament.
(sorry about the double post)
(emphasis mine) In South Africa, due to what I can only call legislative weirdness, when Parliament legalized same-sex marriage it simultaneously also created civil partnerships. These civil partnerships are available to same-sex and opposite-sex couples, and the law specifically makes them legally identical to marriage for all purposes, so it’s not at all clear what the point is.
And the same is true for opposite-sex marriage. If you get married in a civil ceremony, that ceremony can have no religious component. You can choose to have a separate religious ceremony for the blessing of your marriage, or whatever, but the civil marriage ceremony itself will be strictly secular.
From that point on, they’re required to be very, very polite to each other. 
Yes, but in some locations, and I think this is what ctnguy was referring to, while a civil ceremony will not have any religious validity or religious trappings and an exclusively-religious marriage will also have no civil validity, some religious ministers (those whose organizations have the proper paperwork) are authorized to perform religious weddings which count for civil purposes.
So,
get married in front of a civil officer: does not count religiously,
get married in a religious ceremony: depending on whether that religion’s officers are accepted as civil officers for purposes of witnessing marriages, and on whether this double character of religious weddings is compulsory or optional, the marriage will or will not count for civil purposes.
For marriages, this varies considerably depending on the laws of the country. For instance, in Canada, a marriage ceremony is required; the licence by itself is not enough for the couple to be married.
The International Brotherhood of Protocol Officers and Etiquette Columnists is a good example of a civil union. 
But for an opposite-sex marriage, you can have a religious ceremony which also serves as the legal ceremony. You are not required to have a civil ceremony to make it official. For a same-sex civil partnership, a religious ceremony cannot serve as the legal ceremony, and you are required to have a non-religious civil ceremony to make it official.
Again, depends on the jurisdiction. In Canada, in those provinces that have civil unions, you can have both opposite-sex and same-sex civil unions, and no religious ceremony is required.
For marriages, either same-sex or opposite-sex, you can have a civil ceremony, or a religious ceremony, if you meet the requirements of the particular religious group. A same-sex couple who have a religious marriage ceremony don’t need to have a civil “do-over” ceremony.
Again, depends on the jurisdiction. When Mrs Piper and I were married by a judge, her Maid of Honour gave a bible reading (the Corinthians passage on love, of course).
By the way, I’m not trying to be snarky by giving these counter-examples. The laws governing the solemnization of marriage vary tremendously, between civil law countries and between common law countries, and also even within common law federations, such as the US. The same goes for civil unions.
The OP’s question is essentially unanswerable, without knowing what jurisdiction the OP is thinking of. All of the answers given above by different posters are likely correct, for a particular jurisdiction; but it’s not possible to give a uniform answer that applies to all jurisdictions that have civil unions.
I should have made it clear, but I was talking specifically about the UK. That thread of posts was prompted by this post:
Fair enough - but it illustrates that there is no single answer to the OP.