Because they're not normal. People say there's no such thing as normal. To an extent I agree, but mostly I don't. There is a 'normal' in humankind. Those who are happy enough going about their business without causing intentional harm to others, those who play by the rules and can love and respect and appreciate others. They're normal. These normal people make mistakes, and have faults. We all do. Normal or not. But what the hell is 'normal' about taking a man or woman and forcing yourself on them? I'd guess that most people really do think they're pretty normal when it comes down to it, we might think we're a bit odd or have quirks etc. But quirks and oddness in personality don't cause you to violate someone's person in such an utterly sick and pysical as well as emotional way.
There's a normal alright. Look at who fill up our prisons. They're not normal. We are not the same kind as those (not including people who may have simply made a mistake and other exceptions to the rule and hence end up in jail). We don't rape and murder and think nothing of it. We don't enjoy seeing genuine and real harm inflicted on anyone.