>>149736934
>Has there ever been a supervillain that stole the hero's love interest and had a full on romantic relationship with them
has unfortunately happened multiple times to Spiderman in the past. This was what Sins Past originally was (gwen stacy's virginity taken by norman osborn who impregnated her with twins she gave birth to in their european love nest before her death, after peter never got past second base with her, thankfully recently retconned away to have been a Mysterio illusion tricking Norman)
then Black Cat shacked up with the Foreigner, banged him while still Peter's roommate and lover, helped him frame Peter for murder, and then said peter's dick wasn't as big. This was back in the 80's.

oh and famously the whole Terra and Slade thing in Judas Contract, for a DC example