Who the hell had the idea to legalize interracial marriages and relationships? Whoever did that...

You mean, who had the idea to outlaw them? Interracial marriages were "legal" throughout human history until a bunch of racists decided to outlaw them. However, it was common for white slave owners to rape their Black slaves and often the children of these unions were also enslaved.

Fun fact: the modern anti-abortion movement was started by segregationists like Jerry Falwell, when they pivoted to opposing abortion rights after segregation became unpopular.