JK Rowling is pretty good at showing us different kinds of wizarding relationships; from the normal Molly and Arthur, to the taboo relationship between metamorphmagus Tonks and werewolf Lupin, to the medieval relationship of Lucius and Narcissa.
Here are five complicated wizarding relationships that may have flown under the radar.
This relationship is the type of relationship that is not only toxic for the two involved, but for the entire community. Revealed to us recently by JK Rowling on pottermore.com, Dorcus Twelvetrees, a witch, was enamored of Bartholomew Barebone, a no-maj and scourer descendant. To make a long story short; Bartholomew learned all he could about witches and wizards before making his move on the wizarding community.
The fallout from this relationship was so bad, President Emily Rappaport of MACUSA had to institute "Rappaport's Law"; banning any fraternization between wizards and no-majs, and anyone that was being an idiot was henceforth known as a "Dorcus".
Find more information on scourers in "Seventeenth Century and Beyond" by JK Rowing.
This relationship created one of the darker wizards of the series, Severus Snape. We don't know much about this relationship, but form what we do know it wasn't happy. Eileen Prince was the captain of the Gobstones Club at Hogwarts School of Witchcraft and Wizardry before, for reasons unknown, marrying muggle Tobias Snape.
The know that the relationship was not happy because of Lily and Snape talking about the latter's parents constant arguing. Interestingly, Snape's friendship with Lily may have been why he decided to recognize his muggle blood and call himself "The Half-Blood Prince".
Another wizard/muggle relationship, Lyall met Hope while rescuing her from a boggart. It was a happy marriage and the two produced a son, Remus John Lupin.
After incurring the wrath of a werewolf, Fenrir Greyback, Lyall son was bitten and cursed forever. Lyall and Hope hid Remus from society until Albus Dumbledore (perhaps in a sort of penance for Ariana) came up with a plan that would allow the former to attend Hogwarts.
Never mentioned by name, Mr. Thomas; a wizard, married his muggle wife and had a son, Dean Thomas. Mr. Thomas never told his wife that he was a wizard, and seemingly abandoned her and Dean.
Mrs. Thomas remarried to a muggle man, and Dean only found out he was a wizard when he got the letter telling him that he had been accepted to Hogwarts. It was only then that Mrs. Thomas began to suspect that her late husband was a wizard.
Revealed by JK Rowling and in notes given to Chris Columbus; the director of "The Sorcerer's Stone" and "The Chamber of Secrets", Mr. Thomas never abandoned his family, but was killed by the death eaters when he refused to join them. The movie may have changed his mother to a witch to avoid confusion.
This relationship was the driving force behind, arguably, the darkest and most evil wizard of all time; Lord Voldemort. Perhaps the strictest advocate of inter-family marriage, the Gaunt's constant cycle of violence and cruelty caused Merope to be barely able to use magic and may have contributed to her falling for muggle Tom Riddle after seeing his starkly contrasting relationship with his girlfriend at the time.
After seeing her father and brother imprisoned and being free for the first time in her life, she made a dire mistake; she enslaved Tom Riddle with magic, just as she had been enslaved to her family, and had a child with him.
After becoming pregnant, Merope lifted the spell; hoping that Tom would now love her in return. Merope was wrong and Tom fled despite her being pregnant with his child. Merope died after giving birth to her son, Tom Marvolo Riddle, at Wool's Orphanage. Her final wish? That her son look like his father who would never love her.