Theodore Sturgeon, one of defining authors of the contemporary genre, described it as "...a story built around human beings, with a human problem, and a human solution, which would not have happened at all without its speculative scientific content."
Basically, if you feel your site falls under these descriptions, it belongs here. Games which lean more toward fantasy or games which include other themes besides science fiction should be listed under one of those other categories. Fantasy, Social, or Other Themes. Games using any of White Wolf's "World of Darkness" themes, even when used with a futuristic setting, belong under the category for those games.