Morality

Rape is an already loaded term from a moral nihilist perspective. Rape means that the victim did not prefer it, by definition. At that point, the opinion you have of the victim, your opinion of how bad rape is, the perpetrator, the risk, etc would be the most relevant for deciding action even if you are a moralist. If you witnessed someone getting raped, but the victim is also the person who murdered your wife, that is definitely something to consider. Additionally, claims of rape will still need to be met with evidence, and ultimately still an individually guided judgment. Anyone could go ahead with consensual sex then claim it was rape later, leaving the rest of us to decide how to proceed with that claim. 'But you have no defensive wounds? Oh yeah he was really strong so I just let him rape me, I promise I'm telling the truth and I promise I really didn't like what he did to me.' You would still have to -decide- based on your own preferences what evidence for or against a rape actually exists before you can even get to the question of what to do to the rapist, if anything.

Looking at the original post, I believe your last question is actually the answer you are looking for. The difference between preferences and morals from the perspective of a moralist is obligation, where preferences specifically hold no claim while morals obligate persons to act or not act a particular way given only a circumstance. To a moral nihilist there is no distinction, there is only preference, where moral obligations are divorced from the circumstances. Your second to last question misunderstands moral nihilism, as 'wrong' or 'right' isn't attached to a circumstance. Instead, the burden of appropriate action comes from a consideration of your own preferences + the circumstances + your knowledge + what you come up with as a remedy, if any, given that you perceive it as a problem.