More specifically, let's talk about sex as it pertains to time travel movies. You're already saying "Well, Gnoizic, the reason you're not getting laid is because you're discussing science fiction." While true, that's not the point. In movies involving time travel (a la Back to the Future), someone jumps through time and they ultimately end up making a splash in an era they don't exist in. One problem that doesn't get any mention stems from sex. Nothing to see on camera, but plenty of unexplored ramifications of tampering with the past. As we know, sperm is not a permanent entity in a man's body. Even unused, it will die off in its own time. We also know that millions (or hundreds of millions) of sperm cells can compete for one egg at any given time. You exist because you survived the crapshoot of reproduction, congrats! Combine the two facts, and we're starting to see a misstep in time travel movies. Namely: Why are we even seeing the same actor in the future? That person very likely no longer exists. For example: Marty McFly goes back in time. He disrupts his parents' romance, but he fixes it by the end of the movie. In the end, George McFly is less nerd, more success story, all because of his son. What does this do? He still gets married to Lorraine, right? Yes. But if Marty showing up causes them to have sex and conceive a child at any different time than they already were, or under any number of other circumstances, that whole reproductive crapshoot starts all over again, with likely different results. Let's say Marty's dad's success lets him and Lorraine go out for a nice night on the town, or even vacation in another state. Or go have a nice dinner. Anything that prevents the original scenario from happening at the exact same time (in this hypothetical: lamer George and Lorraine have nothing to do tonight, so they stay home and boink on the couch.) will lead to any number of variables changing in reproduction. The little swimmer who would've been Marty's older brother may, in this case, get lost in the shuffle, or Lorraine just doesn't get pregnant this time around. That little swimmer's gone. A new one, with different traits, may take his place. Same parents, and still Marty's brother, but he's going to be an entirely different person. An even worse ramification: Marty himself could become someone else, while the original is just another swimmer lost in the shuffle. He ceases to exist, and an entirely different person is born. Sure, keeping the same actor in and pretending like George and Lorraine still had the same exact sex at the same exact time in their more successful lives is easy on the audience. But of all unaddressed time travel mishaps, this is one that could easily take the cake.