My take would be that I would also be sure to use the "calc" Date Qualifier against the Date of Birth in this case - I would then be quite happy to regard the Marriage Cert as Primary evidence unless or until I had better information.  For me, Secondary usually means e.g.plausible information from someone else's Tree, not yet verified or e.g. my Grandmother's handwritten list of the Dates of Birth of her siblings (since verified as corect), Regards Steve