One of my favorite TNG episodes is First Contact. Not the movie, mind you, which certainly is a wonderful movie (second only to The Undiscovered Country in my book), but the television episode in Season 4, where Riker is hurt and inadvertently introduces the leaders of a planet who thought they were alone in the universe, to the vastness of the Federation.
I love this episode because it could easily play out in real live. Were there to be alien encounters with leaders on earth (like Chancellor Durken in the show), it's not unreasonable they would determine that we were "not ready" and keep the encounter a secret until we had evolved a little more. It was a mesmerizing concept to me, especially at the young age I saw it first.
My question is about sex. Riker, when attempting to escape from the hospital sleeps with a nurse named Lanel in exchange for her help to gain his short-lived freedom. This happens on Star Trek all the time, and that's fine. My question is more about biological differences. How is it possible that Kirk, Riker, and so many other Star Trek male characters have sex with so many aliens? How do they explain that sexual construction is compatible when other qualities are so vastly different?