An important concept in medieval romance, where you are in love with someone unreachable, where to actually sleep with them would invite disaster, so instead you pine after them, serve them loyally, write poems and songs in their honour etc. This tradition became so popular that it became a status symbol for a noble to have his wife admired by a troubadour.
In his book De Arte Honesti Amandi Andreas Capellanus sets forth the rules of courtly love. It is not entirely certain wether he is serious or simply making fun of literary custom. His rules include such things as Marriage is no excuse for not loving and Once love has become public, it rarely lasts.
Examples of the medieval romantic disasters from doing something with someone you shouldn't include Lancelot and Guinevere in the story of King Arthur, Abelard and Heloise, Henry VIII and a number of young ladies, and his daughter Elizabeth I and a number of young men.
Remember, friends don't let friends sleep with Tudors.