Mary, Queen of Scots:
1. Not Middle Ages
2. Protestants (yes this makes a difference)
Matilda of Flanders, Queen of England:
There are legends she refused the marriage and was raped. But this don't seem to have any base. But this marriage was firstly illegal from Papal site because of consanguinity... just William didn't care.
From what I recall there's also a story that says that when a certain duke of Aquitaine was dying, during a pilgrimage to Santiago de Compostela, he asked a fellow pilgrim to take the journey back to Bordeux for him under disguise carrying knews of his death, but that he (or she) should ONLY inform the chaplain, for the duke feared that after knowing his daughters were fatherless, someone would kidnap and impose a marriage to his eldest daughter, Aliénor (who would later became the famous Eleanor of Aquitaine, queen of England). The bishop -is said- took the heiress to a monastery and sent a letter to the king of France, asking him to put them under his protection before bringing the news to the rest of the court. It sort of worked out, sort of backfired. But that's another story.
That, however, may have less to do with a forced marriage not being contestable and more to do with the fact that at the time Aliénor simply had no relative in France her father could be sure would contest said marriage if it happened, so she was a really tempting prize. The house of Poitou was heavily comprised of women married off long ago, distant cousins who were known for taking bribes and the diseased duke's bastard half-brother, who was far away on the Holy Land.
To be honest, her relatives would probably be more likely to try taking her lands to themselves than to defend her, so that may have being his real concern and the protection from kidnapping could have being just a better story (and a safer one to tell around, since it doesn't incur in dangerous acusations).