It isn't very often that a scholar has the opportunity to discuss knowledge from other worlds with someone who actually came from one, but Twilight is one of the lucky ones. A being called a Pokemon, identifying himself as something termed 'Mewtwo' has ended up in Equestria, and Twilight is bound and determined to learn whatever she can from him - even if it means asking embarrassing questions. After all, what sense do Egg Groups even make?

Aside from gaining new information, Twilight also hopes to form a bond of friendship with the intellectual Mewtwo - named Grimalkin - based on their shared scholarly nature. Their discussions range from the history of both of their worlds, to matters of physiology - this particular talk being focused on the curious matter of Pokemon reproduction.

This fiction may yet tie in with other stories of mine as a side story, but it could potentially stand on its own.
As reproduction is only alluded to, rather than described in great detail, I believe this rates as Teen.