Formally, slave marriage was discouraged by plantation owners and slave masters back in the days of slavery. Some slave owners on the other hand, encouraged marriage among their slaves because they felt that marriage would make their male slaves less of a "flight risk". Slaves were however not barred form having children and in fact, were encouraged to do so because they would then become slaves of their parents' owners. The slave masters were also able to sell those slaves who were particularly fertile for a slightly higher fee.
Slave weddings were typically officiated by the slave owners and were carried out with very little pomp and circumstance. The couple might have been asked to hold hands while Bible verses were read, they were then declared married. Naturally, honeymoons were not an option and in fact both husband and wife were expected to resume their duties as normal the following day. Marriage between slaves was not legally recognized.
If slaves from different plantations fell in love, then at their owners' discretion they might be "allowed" to get married. Conjugal visits were arranged by their owners but there was no guarantee that either the husband or wife would not be sold.
Even for slaves that chose to practice Christianity, it was rare for plantation owners to honor their marriage vows. Thus, even when they did try to establish formal family units, neither their owners nor the laws of the land recognized them or even took them seriously.
"Jumping the broom" was a part of wedding ceremony among slaves. It symbolized the official joining of the two as one amongst the slaves.