It's not quite clear exactly whose fault it was. It might have been James MacPherson and his Ossian books, or Thomas Percy and his Reliques of Ancient English Poetry, all published in the 1760s (though to be fair, these publications were at first much more popular and influential on the continent). It might have been Horace Walpole and all the other writers of gothic novels. It might have been the antiquarians who started to become interested in medieval life and medieval objects in the late 18th century.
Sir Walter Scott, whose historical novels filled with knights, adventure and romance (Ivanhoe anybody?), certainly has to shoulder a large part of the blame.
Whoever or whatever it was: from the late 18th century onwards the British rediscovered the Middle Ages--with a vengeance! At first it was all quite innocent: one enjoyed a gothic novel or two; an old ballad, perhaps; if you were rich, you might have fancied putting some fake ruins in your gardens, or why not have your country estate remodelled in the neo-gothic style? Add to that portraits of people in (fake) armour and other sort of medieval clothing, and the odd collector of medieval knick-knack.
But then somebody got it into his head that these outward trappings were simply not enough. No, on top of these one ought to revive the spirit of the Middle Ages, the great ideals of "the days of old when knights were bold" (or at least what were thought to be the ideals of the knights of old)--in one word: CHIVALRY. As a result of this, the ideal of masculinity radically changed in the course of the 19th century: a man had to be tough, brave, show no sign of cowardice, save / protect women and children, and meet death without flinching.
The Victorian Age saw an immense production of history books, ballads, novels, music and paintings that celebrated this chivalric ideal and thus taught it to whole generations of boys and young men. It proved to be so influential that at the beginning of the 20th century the founder of the boyscout movement, Baden-Powell, claimed that the boyscouts were the modern descendants of the knights of old:
You Patrol Leaders and Scouts are therefore very like the knights and their retainers, especially if you keep your honour ever before you in the first place, and do your best to help other people [...] Your motto is, 'Be Prepared' to do this, and the motto of the knights was a similar one, 'Be Always Ready.'
The knights of old were particularly attentive in respect and courtesy to women. When walking with a lady or child, a Scout should always have her on his left side, so that his right is free to protect her.(Both quotations are from an abridged version of Scouting for Boys, which was first published in 1908.)
Protect her from what? we might ask. For even in the early 20th century it wasn't all that likely that a dragon would hide behind some bushes in the park, waiting to snatch a tasty little woman. As you can see from this example, the medieval revival and the romanticized version of the Middle Ages offered the opportunity to glorify old genderroles--this became particularly important towards the end of the century when these old genderroles came under increasing attack and women started to demand more rights.