The expression “to jump over the broomstick” isn’t very common now, though it is used occasionally by writers.
The dictionaries are not very explicit, saying merely that it means to go through a mock marriage ceremony, in which both panics jump over a broomstick.
During the times, a few short centuries ago, when marriage laws were not very stringent and not at all uniform, a man and woman might go through the formality of publication of the banns but might live together as man and wife without waiting for the sanction of the church.
This, through a popular superstition of the times, was thought to be quite proper and legal if both parties jumped over a broomstick. But in Scotland, broomstick or besom now means a prostitute, and in France rotir le balai, literally, to roast the broomstick, means to lead a dissolute life.
So perhaps those who “married over the broomstick” did not always or long remain in wedded state.
