What makes rape immoral is that rape harms the victim in terrible ways. The victim feels pain, loses freedom, feels subordinated, and so on. These harms are not justified by any benefits to anyone.
My thing is that rape is objectively wrong because it's wrong. I've read that many men rationalize rape by thinking that their sexual prowess is so great that although a woman is initially resistant and in fact says "no", he can change her mind. Such men don't think they are causing women to suffer. In their twisted imaginations they think they are doing a woman a favor. How do you convince such a person that what they are doing is wrong?
Biblically, in ancient Israel, that is why rapists were executed. I don't think that executing rapists is what we need to do now, but I think just assuming that people know that its wrong to cause pain and suffering for others isn't going to work.
Atheism and Rape - Walter Sinnott-Armstrong Elucidates | True Freethinker