I live in Canada, and I was considering leaving this forum, as we do have a law that prohibits what is defined as child pornography. I reread very carefully the section 163.1 (1) c):
"(c) any written material whose dominant characteristic is the description, for a sexual purpose, of sexual activity with a person under the age of eighteen years that would be an offence under this Act" (emphasis mine)
In other words, unless I am mistaken, I can write fictional stories that involve sexual acts with or even raping of underage characters, as long as it is not the dominant characteristic of my story. For example, I tell the story of revolted gladiators who overrun a Roman villa and rape all the women and girls there. Underage sex is not the dominant element, but it's part of it.
In another story, I tell of the ordeal of a group of Briton university students who find themselves entangled in political unrest in Uganda. That's fine, since most girls are 18-25, and there are only a few 17-yo first-year students.
What I think I cannot write is, for instance, a gang of angry Latinos storming a dorm where some high-school students are staying. That would make underage content the dominant characteristic.
I can always post a warning and then not mention the exact age of the characters, and just say "she was a gorgeous teen queen" or something along those lines.
Thus, I'll stay on this forum, for I enjoy very much writing these stories. All I need to do is staying cautious of the underage elements in my stories.