Some people do not understand the idea of discussing human values as if you were designing a computer program or experimenting with the living conditions of rats in a laboratory. When you ask "What, exactly, makes rape 'wrong'?", they do not think you are inquiring as to the nature of rightness or wrongness, and how those values come to be assigned. They think, instead that you are suggesting that rape should not be illegal. They strongly believe that rape SHOULD be illegal, so they respond harshly. The harsh response is due to poor socialization, which comes into play because they do not understand the conceptual basis of your question.
If we posit a universe without a god, and therefore no morality, we have a dilemma trying to choose a basis for telling good actions from bad ones. Indeed, without a god, would there even BE "good" actions or "bad" actions? A great deal of philosophy is taken up with this very argument.
Many people fall back to the idea that civilization (people living and working together for their common good) is a non-god-defined test for what's good and what's bad. If the practice disrupts the civilization, then it is bad - that's how this philosophical school would address the question of rape. Rape certainly disrupts civilization - no doubt about that - so it would be rational to accept its "wrongness".
However, not everyone is pleased with this sterile and structural approach. They feel that it ignores trauma and injustice, so they seek other philosophical models. There are also those who reject the civilization test, too, but for different reasons. They seek a less-situationally-dependent test for rightness and wrongness. If you are being asked to respond to a quote saying rape is NOT wrong, the speaker is probably thinking along those lines, claiming that, if no god exists, then there is absolutely nothing that provides a basis for saying any act is bad or wrong. It isn't stupid. It is just a point of view in a philosophical context.