The two reasons I've heard are the two Papi listed. Those two seem to also make sense to me, as I've personally seen both occur. I've seen a pissed off wife calmly walk in in and then abruptly start punching her husband right at the bar without warning. I've also been propositioned for OTC by what I thought was just another customer who happened to be female.
What GMD said is also very true. However, there is an exception for situations where the its done to protect the well being/safety of either one of those classes or the remaining patrons. One could argue that the two justifications given actually apply to the well being of the other patrons. I'm not going to look it up, but it seems totally reasonable that some wife could come in and cause a ruckus resulting an a 3rd party being injured. If that happened somewhere, it would serve to support the theory. If the 3rd party sued for damages and won, even more so. Of course it starts to fall apart if you consider nontraditional relationships, but then you get into an even more subjective area.
Anyway, I can't see one of the reasons for keeping out unescorted females out having to do with licensing requirements. If enforcement isn't capable of distinguishing employees from patrons they're not capable of enforcing and any halfway decent lawyer would easily win and likely get their fee's covered by the state as well. But, I am not a lawyer or anything close.