Francois, I think that the grandfather forcing himself on the granddaughter is immoral. Besides the fact that he is forcing himself on the only other human that's alive, he's also not really looking at the big picture. Any number of things could happen as a result of his rape.
She could kill him and then herself. Human race is still gone. She may not become pregnant...will he rape her again? She could die in an accident...human race is still gone and he never had a chance to make peace with his granddaughter. She will probably hate him for the rest of their time together, maybe run away from him...who will teach her to survive once he's gone? He might think that his motives are unselfish, but by not considering her or what could result from his actions, he's acting selfishly and hurting his granddaughter in the process.
As to your question regarding whether the human race should survive in that scenario, I'd have to say NO. If people have obliterated themselves and one of the only two people left is a rapist I think the universe would be better off without us.
Here's the problem that I'm having with the rest of this discussion. Some of you are asserting that if a person commits an act such as rape or murder under circumstances that could possibly be understood or justified that proves 100% that there are no moral absolutes. I don't think that conclusion can be reached by the scenarios introduced in this discussion, including those about children having sex with adults in ancient Roman times.
If someone is in a position where they have to choose the lesser of two evils, that in no way implies that the evil they chose is the moral thing to do. The fact that they have chosen one bad thing over another does not make one of those bad things moral and right. The choice the person makes when confronted with such a choice is not even a reflection of their morality. Mortality perhaps, but not morality. If a moral person unwillingly commits an immoral act I don't think that they then feel that the act has become moral by virtue of their committing it. In fact, they would probably feel guilt or regret for it even if the act is somehow justifiable. I seriously doubt if they would think that they committed a moral "right".
I really think it's a stretch to assert that moral absolutes are nonexistant just because a person in the midst of a difficult situation might choose to do something immoral when that seems to be the only option. Morality is probably the last thing on someones mind when they or their loved ones are faced with physical danger, frightened and threatened. Most people will just react and deal with the moral issues later.
Just because someone makes a decision to commit an immoral act does not mean that act is no longer immoral or the moral implications of that act don't exist. Therefore, moral absolutes do exist just as immorality exists. One doesn't negate the other.
Other than that...what plum said.
