Merope Gaunt and Tom Riddle Sr.
We may never agree on whether Severus Snape is a Byronic hero worthy of Lily’s love or just some greasy weirdo, but I think we’re all of the opinion that Merope Gaunt and Tom Riddle Sr. are a certifiably messed-up couple. Something like 100% of people agree on this.
Merope has a pretty terrible life. She has an abusive father and a brother who keeps nailing snakes to things, so it makes sense that she would be attracted to someone who's so far removed from this world of squalor that he's practically on another planet—Tom Riddle, the hot Muggle boy next door. But she uses a potion to make him love her back, and that’s super disturbing. You can’t have a healthy, functioning relationship when one person’s under the influence of a love potion. You just can’t do it.