Men are faster and stronger and have higher fortitude, there are very few positions where a woman might be useful.
Possible positions:
WR: but considering they're usually shorter/slower/weaker, the only way this would happen would be if the woman in particular had AMAZING hands and almost never dropped the ball and would catch any pass thrown her way.
Kicker/punter: the only "realistic" position. A position largely determined by technique and finesse vs. strength/speed. Women are more flexible and could possibly excel very well at punting or kicking. Even though male kickers might have a stronger leg and thus able to kick it further, if a female kicker (similar situation as with WR) can be super efficient with her kicks it could be worth it - you can always have a 2nd kicker just for the long ball.
However the odds are just way too stacked. Even if there was a woman "tough" enough to do it, chances are she wouldn't have the skill/tallent/natural ability to be a good player (or at least not as good), so if it isn't one thing, it is most likely another that keeps said woman from playing. From my example the woman would have to be down right unbelievable, never dropping passes thrown her way (as in decent passes that she can actually get her hands on), or a kicker who can hit 95% of her kicks no matter what (even though her long might only be 48yards or whatever) - it is just very unreasonable to think that something like that would happen - possible, just very very very unlikely.