Jim Guest asked "So does that mean that underage sex is ok?"
That's a bit of a non sequitur! I can't really see why, saying our understanding of death has improved over time, suggests that under age sex is okay?
To answer your question, I think there is a huge difference between two, mature for their age, teenagers having sex and a case where there is a large age difference. But, in general no, our improved understanding of death doesn't mean that under age sex is okay.