The EU1 Instructions document states that
When submitting documents please note the following:
1. All documents submitted must be photocopies.
...
3. Marriage certificates and birth certificates should be photocopied in colour and should include any Apostilles or legalisation
...
So I think this means you should have the original available if it is requested. I would expect that civil marriage certificates issued by a U.S. city, county, or state should already have an official stamp or watermark on them to indicate that they are "legalised". So if this is what you mean by registered, the document is likely OK as long as it is an original document.
BTW, I think the term "original" is often misunderstood. It does not mean a document that was issued at the time the event happened (birth, marriage, etc.). It means an original document that was issued by an official agency - and it could have been issued many years after the fact. For example, I recently obtained my grandfather's birth certificate from Ireland. But the document I received is still an original, because it was issued by the government - even though it was issued 120 years after he was born.
http://www.inis.gov.ie/en/INIS/Form%20E ... eaflet.pdf
