Their love was not an epic love. They were not like Andromeda, who was disowned by her Pureblood family after falling in love and marrying a disgusting Mudblood.
Unlike Ted Tonks and Andromeda Black, Lucius Malfoy and Narcissa Black were an acceptable pairing in the eyes of their society. Both of their Pureblood families were ancient, prestigious, and wealthy. Even among other Purebloods, they were royalty.
On the surface, there was a simplicity to Lucius's and Narcissa's relationship. It was assumed by most that they had only married each other because they had been arranged and therefore it had been expected out of them.
And indeed, there was no changing or denying the fact that Lucius and Narcissa were both Purebloods and that neither had been forced to endure any epic struggles in order to become husband and wife.
There was more, however, to Lucius's and Narcissa's relationship than just the fact that they were both Purebloods. They didn't kiss each other just for the sake of kissing or giggle like love-struck fools whenever they looked at each other. They didn't touch each other intimately for everyone to see. They never made an epic show out of their feelings for each other. They never felt the need to do any of those things, as they found such behavior to be common and therefore the type of behavior that would be expected out of a Muggle but never out of a proper, respectable Pureblood.
However, when Lucius was arrested and imprisoned in Azkaban, the true depths of their love was finally revealed.
People would have perhaps looked more kindly at Narcissa if she had declared that her husband had been and would always be nothing but a monster to her. Narcissa, though, did not betray her husband when he was at his lowest point in life. Instead, whenever someone dared to make a snide comment about him, she quickly made a snide comment to that person in return.
Narcissa also never blamed Lucius for all that she suffered because of his failure in the Department of Mysteries. Instead, she continued to give him nothing but support and comfort. Whenever Lucius needed a soothing hand to calm him down, Narcissa was always there to offer that hand.
Lucius and Narcissa never made any epic speeches about their deep and eternal love for each other. Lucius in particular did not even like the word "love." However, when it truly mattered, Lucius and Narcissa were there for each other. They did whatever it took to keep themselves and their son Draco alive.
Lucius and Narcissa didn't care about the world. They especially didn't care about what the world thought of them. However, they did care about each other. They just didn't feel the need to make an epic show out of it.