Simon17,
If your son asks you, "Is rape really wrong?" are you going to say, "It depends on who you ask. Cosmically, it's insignificant" and then talk about societal disadvantages and the lack of evolutionary advantage that would come from allowing rape?
Or are you just going to say, "Yes, son, rape really is wrong"?
What I tell my son is irrelevant.
To answer your question anyway, of course I'm not going to tell a little kid what I just typed. Just like people don't tell their children the gory details of "where do babies come from." Just like even adults make up stories about where the dead go and how happy they are and such. Just because we lie to little children or to ourselves about what we would like to be true or what we feel should be true... it doesn't make it true.
Beyond that, in our society, the way we have developed it IS wrong. So there is actually not an inconsistency in my saying to a child that rape is wrong. Is it an absolute TRUTH that rape is wrong? That I can't say, but it doesn't effect one iota of how I function as a human being.