Dating and marriage in middle ages
In secular law, clandestine marriages were frowned upon because of the difficulties they could bring into determining inheritance.
A marriage which was not public (or at least witnessed by someone other than the couple) could lead to claims on property by other relatives upon the couple's death....which, of course, meant that it was left to the heir to prove that the marriage was valid.
A plight-troth could be broken, but only with the agreement of both parties.
Another bar to marriage was a monastic or religious vow.
This is also what is behind the presiding priest's question to those attending a wedding as to whether anyone knew of reasons why a couple could not marry.
There were a good many reasons why a couple might not be permitted to marry. If a couple were too closely related (usually within the third or fourth degree), the marriage could not occur, though the Pope could issue dispensations for second cousins or less, and occasionally for first cousins.
This was to insure that both parties were willing, that there were no grounds which would invalidate the marriage.
(Some of these grounds could be overlooked with a papal dispensation).