I've been researching ADAMS of VA for 10+ years and the John Adams-Katherine Napier "marriage" was already around at that time. This John ADAMS does not appear to be my line but I keep bumping into them and their descendants in my research.
If you check the LDS familysearch.org website, you will find no less than 19 pedigree resource files with this marriage listed. Then familysearch bundles this data into a CD and sells it! The data on my own ADAMS line at familysearch.org is horribly wrong in several different pedigree resource files. Sadly, we are powerless to correct it and the unconfirmed data is spread even further with each CD sold.
In my 10+ years of bumping into John ADAMS and Katherine NAPIER, I've never seen a source document noted for the marriage. However, I've never seriously dug for a source, either, since this doesn't appear to be my line.
Looks like a good old-fashioned dusty court archives dig is in order to prove or disprove this marriage! ;-)