In pure blood society, relationship rules are strict.
Especially the one about 'No more than one girl or boy at one time.'
So Lucius Malfoy and Bellatrix Lestrange were breaking that rule.
Both were married.
One had a child.
The other was hanging onto three men.
But their motto was strict;
"We'll go into the fire, we'll be death defying, and we'll have the time of our lives!"
If they didn't get caught, that was.