I've always thought the most horrible way a villain can mess with their heroic archenemies is by seducing and sexually conquering, by force or by consent, the love interest of their hated foes. Here's a list of baddies who did just that!
Seduced and had sex with Betty Ross while she was angry at the Hulk during her Red She Hulk phase
An utter soldier jerk, habitual archenemy, and rival to the Hulk who managed to get Betty Ross to marry him willingly. Poor Betty just has terrible taste in men.
This one not only raped Betty Ross in her sleep to magically conceive the entity "Daydream", but killed Betty Ross and Bruce's unborn baby. Heartbreak and Rape? Poor Betty can't catch a break.
Does this with alarming frequency against Wolverine, having raped and murdered one of his first true loves, Silver Fox
As Original Sin will prove, Norman got busy with Gwen Stacy when she was depressed over having troubles with Peter and had her give birth to his children. He also seduced his own son's wife and tried to knock her up with a son to be Harry's replacement. Norman is NOT a nice man.
Had a romantic relationship with Terra while she was in a relationship with Beast Boy during her time as a mole against the Teen Titans.
Raped Casca, Gut's true love, before his eyes in one of the most graphic and horrific displays of torture ever put into ink.
Raped Macleod's wife centuries ago during their first battle and still taunts him about it centuries later
Captured and forced Corsair's wife, Cyclops'/Scott Summer's mother, to be his sex slave before killing her before Corsair's eyes.
Sylar possessed Matt Parkman's body and slept with his wife. He later bragged that she enjoyed Sylar's version more than she ever enjoyed Matt's.
Got Nathaniel Adams imprisoned, experimented on, and sent ten years into the future as a quantum superman. Meanwhile he seduced his lonely wife, later married her with consent, and raised Adams' children as his own all while being an utter military asshole. Was Adams' wife that desperate?
They killed Eric Draven and raped and murdered his fiance Shelley, causing him to resurrect as the Crow to enact vengeance for their evil.
In the Pre-Crisis verse, Luthor briefly seduced Alice Spencer, Perry White's wife, while Perry was overseas reporting on a war and thought to be dead. Luthor used the opportunity to sleep with her and conceive a son, which Perry thought was his until he later uncovered Luthor's affair.
After the First Incursion and the beginning of the Second Secret Wars, Doom rebuilt the remains of Earth into his own Battlefield and, to spite Reed Richards, recreated his wife Susan Storm to be his wife and lover and took custody of Reed's kids.
Seduced Dick Grayson by the shape-shifting in the form of his girlfriend Koriand'r/Starfire and became a large factor in them subsequently breaking up for good.
Anton Arcane, Swamp Thing's primary nemesis, famously did this to Abby, his own niece and Swamp Thing's lover, by possessing the body of her husband.
One Nasty Mother. She actually once created a new identity as an X-Men student known as Foxx, in order to seduce Gambit and break him and Rogue up for shits and giggles and to set her up with a work buddy. Rogue is her adopted daughter!
Left the Legendary Kenshiro in the dirt bleeding after stealing his beloved Yuria and giving him his seven scars, kicking off Kenshiro's journey of revenge and legendary badass status.
