Subjective Morality?

[Belacqua]

You could no more be convinced that anything was good or bad if you lacked this sense than you could be convinced that I was blue if you lacked eyes.  

A naturalist would point you to whatever cluster of natural facts establishes that rape is among the category of things we refer to when we use the term.  If you can't see those, then, similarly...you lack necessary faculties

I expect you know that famous thought experiment about Mary the Super-Scientist...? A quick reminder:

Mary has a dozen advanced degrees in various sciences. She knows absolutely everything there is to know about the physics of light, how it reflects off of objects etc. Also she knows everything there is to know about how our eyes and brains perceive and represent light in the mind. However, she is color-blind. She has never experienced the difference between red and green in her own mind. Is there something about color she doesn't know? 

What you say above indicates to me that there may be a sort of moral-blind Mary as well. She might know all there is to know about what rape is and what psychological effects it has on its victims, but doesn't have a moral sense that it is wrong. 

I still think that what your interlocutors are wanting here, when they look for evidence that morals can be objective, is something that color-blind/moral-blind Mary could describe. Something that is scientifically discoverable about the act even in the absence of our normal mental response. 

Whether the wrongness of rape is real or not, then, would depend on whether people accept that color is real. Some deny it's real just because it only appears in the mind. They say that it has to appear outside the mind to deserve the label "real." But other people will say that a reaction every normal person has to a certain stimulus -- whether a color reaction or a moral reaction -- does constitute a real thing. 

Is this a relevant description of what you have in mind?