WAIT a minute. Backflow WHERE? There is only one exit, and that's the cervix. There IS no opening from the uterus to the abdominal cavity. I can see why these problems are "imagined" - and I find "Even if there were a problem - and there isn't - there still wouldn't be a problem" to be a great example of a nonanswer.
Tampons have positive absorbency, so inserting one will PULL fluids from wherever it can, perhaps substituting for gravity. Hadn't thought of that. Bet they just skip their periods while offplanet, though, must be easier, the way it is down here. And mousethief, what exactly did you date - a woman? (*BG*)