It looks like people are searching for nice, clean answers. Well, biology and evolution is a messy business, there are competing factors to our survival that might never be perfectly balanced (at least, not until the advent of 2-directional mind machine interface and the ability to reprogram the brain
). On one hand, there's a strong desire to reproduced hard-wired in our genetic code by a billion years of evolution, it's absolutely necessary, the species would not have survived without it. On the other hand, we became social animals much more recently and we have evolved to optimize our social situation, but evolution can't just throw out the desire to reproduce, the species would cease to exist. So, on one hand we have a fundamental desire to reproduce at all costs and on the other hand we have the desire to live as a successful social people. It's not unreasonable to suggest that certain individuals in the society will be outliers on one extreme or the other, but on the average evolution has struck a pretty good balance.
Then, through at least the last 100,000 years of evolution we can add to natural selection artificial selection. If you go back far enough, you will see that rape was largely condemned as a violation of the man's (be it the father's or mate's/husband's) property rights and it threatened his genetic line, so he and others of power and influence in the tribe, acted to protect their survival and procreation by killing the guilty party. Over the years, those born with the strongest sexual inclinations, ones that caused them to go outside the bounds of what came to be regarded as socially acceptable, were killed...of course, those with no desire to reproduce may not have been killed, but they were also much less likely to reproduce. The result was that those who were most inclined to justify rape because of a strong and primitive reproductive desire were artificially removed from the gene pool and now we're at the point that, on average (of course there are outliers), we consider it inherently morally reprehensible without having to resort to property rights or a desire to ensure the integrity of our genetic line; and, of course, this has all run in parallel with the evolution of our brains and our capacity to reason and the cultural evolution that this element of biological evolution has allowed. We weren't always like this, this mindset and our modern reasoning capabilities are a result of thousands of generations of evolutionary selection, both natural and, more recently, artificial. But there's nothing wrong with seeing that we are a superior beings compared to our ancestors, that is the direction evolution should take, nor is this 'moral code' (I use the term morality with caution because of the superstitious baggage it carries, but it is the term that most accurately describes this instinct) any less important because of how we gained it, if anything it makes it more important because it has been tested and verified by evolution. We evolved this way for a good, objective reason, it has greatly aided the survival and advancement of our species. To me that seems like a far better reason to conduct our lives according to this 'moral code' than simply because the invisible man in the sky said we should.