You know its funny, Im not married either but I often wondered about this considering its a tradionally Black marriage rite. I found this, its pretty cool.....
"...The practice of couples jumping the broom during wedding ceremonies is an African American custom.
In contrast to many West African traditions recognized and included in African American wedding ceremonies, the practice of jumping the broom is a wholly American addition that developed out of the institution of African slavery in the United States.
Shortly after the introduction of slavery to the country, the right to legal marriage was taken away from the enslaved. Slaveholders considered the Africans property, and as property, they had no rights in the eyes of the law. The owners also feared that legal marriage and family bonds had the potential to lead to organization and revolt.
Marriage rituals were important events to the Africans who came from numerous richly ceremonial cultures. When faced with the loss of the right to marry, many, ingrained with the significance of the marriage since childhood, created new rituals with what was on hand.
Taking vows in the presence of a witness and then leaping over the handle of a broom became the common practice to create a recognized union. Incorporating jumping the broom into modern weddings is just one way African Americans today celebrate their cultural heritage...."
http://en.wikipedia.org/wiki/Jumping_the_broomstick