As a royal Princess Victoria Mary was not actually allowed to marry a member of the regular society.
However, Queen Victoria's daughter Princess Louise married John Campbell, Marquess of Lorne, who became 9th Duke of Argyll.
I always found it odd --she was too royal to marry a British aristocrat, but too morganatic to marry a German prince. Her brother, on the other hand, married a British aristocrat. I guess Lady Margaret Grosvenor's money was perfectly acceptable and ebengeburtig.