You may have the content all planned, written and ready, but what about the honorifics?
I know a lot of people who like to address their wedding cards to Mr and Mrs *insert surname*, because it may be the first time that the bride, at least, changes her name.
(Though not necessarily. Some brides don't; and sometimes the couple both change, as their family names become hyphenated.)
If you really want to run with this for a gay couple's wedding card, then you need to find out first what is happening with their surnames.
If Mr Jones is marrying Mr Brown, then they may decide to both become Mr Jones-Brown. But they may equally opt to stick with the names that they took into the marriage. Very occasionally, you might find that Mr Brown would rather take on Mr Jones's name, so they become Mr and Mr Jones.
You won't know until you ask!
Alternatively, you could address your gay wedding card to 'The Grooms' or 'The Brides'. There is no barrier against adding a lovely adjective, like 'To two handsome grooms' or 'To two glorious brides on their wedding day'.
Safest of all is opting for first names, which are highly unlikely to change at the altar. 'Dear Adam and Steve' or 'Dear Adina and Eve' is a perfect way to address a greeting card for a gay or lesbian couple. Though, obviously, their actual names would be more appropriate!