Themes are suggestions, not hard and fast requirements, and we won't reject a submission just because it doesn't match the others. We keep them around because they're good creative springboards -- 'write a story' is a pretty vague instruction that can be daunting, but 'write a story about monsters' gets the creative impulse flowing in a specific direction -- and because if you've ever walked through a museum, you know how great curated exhibits with a single unifying characteristic can be. However, contributors should never let something as simple as an issue's theme stop them from creating whatever the hell they want. Likewise, loose thematic interpretations are also great; sometimes doing it wrong is doing it right. If you want to see a specific topic/trope/theme/fetish/kink in SSB*B, the absolute best way to guarantee its inclusion is to do it yourself.
When we choose themes, we keep several factors in mind. First and foremost, a theme has to be something open to broad enough interpretation that it can inspire a wide variety of stories; we don't want an issue that has the same story twice, much less a dozen times over. It's better if it's less of a specific story element and more of a general piece of inspiration; for instance, the theme 'Doctors' would suggest a much narrower range of contributions than would 'Playing Doctor' (speaking of 2013). Also, because the themes are basically fetishes, we'd prefer to stay away from broad human categories that lend themselves to stereotyping (e.g., race, ethnicity, religion, disability) and instead encourage writers to be attentive to issues of diversity without mandating their doing so.
Finally, the theme has to be something we haven't done before! To that end, and for your archival reading pleasure, may I call your attention to our back issues, where some of the themes you've suggested may already be waiting for you! Here's a list of the more topic-themed issues we've had thus far, in case they're relevant to your interests (with brief notes where the theme isn't necessarily obvious from the title):
Tales of Magic and Mystery
Animal Kingdom (pets, were-furries)
Position of Power
An Officer and a Gentleman (uniforms, military)
Dress Me Up, Dress Me Down (clothes, costumes, crossdressing)
100 Tales of Horror
Pop! Goes My Heart (music)
Faster Higher Stronger (sports)
A Web Of Intrigue (politics, spies)
He Blinded Me With Science (hit me with technology)
Ninjas Versus Pirates
The Food of Love (complete with recipes)
Fairy Tales and Folklore
To Infinity and Beyond (science fiction)
Wild Wild West (with surprisingly few cowboys)
Hot For Teacher (with surprisingly little teacher/student)
Myths and Monsters
And this isn't even getting into the business of all the stories in the theme-free issues that would have been just as appropriate in one of these themed issues, or stories in one of the themed issues that would have been just as comfortable running in another. Variety is the spice of SSB*B, after all. Thanks for all the suggestions thus far, and please, keep them coming!