Why is it called a “stain”?

from: http://en.wikipedia.org/wiki/Immaculate_Conception

The Immaculate Conception of Mary is a dogma of the Roman Catholic Church, according to which the Virgin Mary was conceived without any stain (in Latin, macula or labes, the second of these two synonymous words being the one used in the formal definition) of original sin.[1][2] It is one of the four dogmata in Roman Catholic Mariology. It is completely distinct from the Virginity of Mary and the virgin birth of Jesus, though it is a popular mistake to confuse them. Mary is sometimes called the Immaculata (the Immaculate One), particularly in artistic contexts.[3]

It is interesting how the word stain is used in this paragraph. It is so easy to make people feel guilty… if you start to use these words…