The unicorn both came to represent Christ, and also began to represent purity and chastity. The idea that unicorns could only be tamed by virgins became a widely held belief, and images of unicorns resting their heads in chaste womens laps, with not so subtle sexual undertones, began to appear in artwork.
Source : http://jezebel.com/5016283/whats-the-deal-with-the-relationship-between-girls-and-unicorns