In other words, according to you, neither the Holocaust nor slavery are objectively bad - they're only bad if you think they're bad? Or they're only bad if certain people think they're bad? Or if a majority thinks they're bad? If there's no objective morality, what - pray tell - are you claiming?

And if there's no objective morality, are you saying that rape might have been ok at some times or in some societies but that rape became immoral later on or only in some places?

I want you to really think through what you're saying; there's no objective morality is just a platitude that people mutter w/o understanding its implications - you're basically claiming that you're fine w/ rape, murder, genocide etc.; your objections to such things are purely contingent. Is that REALLY what you mean? Or are you quickly about to tell me all about the "objective" morality that makes all those things awful all the time no matter what?