They will attack other males presumable to keep them away from potential mates. Female unicorns are the most attracted to the male with the largest horn. Gestation is roughly the same time for unicorns that is is for humans - 9 months. The female then leaves the males territory and secludes herself in the forest for the winter. T hey are also said to be attracted to virgins, which is one way that they were hunted.
Virgins represent purity, and a unicorn is very pure.
By using a virgin, hunters were able to seek out unicorns and take their horns, killing them. Unicorns are also able to detect virgins. If the horn of a unicorn pierces the heart of the woman, she is not a virgin. This method is sometimes said to be applied to trustworthy and sincere people in general, such as in the Xanth books by Piers Anthony.