"Medieval mentality" is a term given to a man's view of dating, relationships, and marriage, in a positive and "Medieval" context. It is derived from the perceptions of a modern man of the 21st century who looks upon Medieval Europe with admiration, believing chivalry was of utmost importance in relations with the opposite at the time. He feels such qualities of chivalry are lacking in today's world, and he wants to apply those same values to his own relationships.
Medieval mentality includes not only winning the heart of your significant other, but also earning the respect and admiration from the significant other's family. Having this mindset means you see your relationship as much more than what it appears at face value. One with medieval mentality views his newly formed romantic relation of strategic importance, be it social, political, or economic. He sees his relationship as a uniting of clans, tribes, etc. in which both parties will mutually benefit.
Turku: Great news, my girlfriend's family treats me as one of their own. I have earned high status among them being the ultimate gentleman. One day they will entrust me to look after their estate.
Tampere: Dude, you have medieval mentality.
Turku: I know, it's totally awesome.