In a competitive 11 a side league I do not think it should be allowed. My main reason for this is that Womens/girls football is developing at such a rate that there are much more opportunities for females to play, unlike there were a few years ago.
I have no problems with females playing in the park/non competitve/charity/5 a side games though I do make the men aware of the fact they need to be careful with their challenges. I have several females that I coach that are better than some of the men I play competitvely with - but it only takes one challenge to end a career.
I live in a rural area, where a few years ago no-one could be bothered with female football. Now there are almost as many girls teams as there are boys. All it takes is a pro-active league/FA for the clubs in the areas to cotton onto the fact that female football is worth bothering with. THe history behind my team is that these group of girls badgered so many people to coach them, and after getting fobbed off a lot, they ended up with me! If there are not the opportunities in your area, then why not do something about it. IF you want some encouragement or advice on how to do this, PM me, and either I or one of the females who I coach will tell you how it was done. 3 years ago in this town there were no ladies teams, now there is 1 ladies team, and 6 junior girls teams scattered round the town.