In some cases, there might be good reason too, in that the service is actually more expensive to provide to one group than to another. Hairdressers may have legitimate claim to charging women more, but only if their cuts require more time, skill, or effort for a more demanding set of customers. In Europe, insurance companies charged women more claiming that because women live longer it's not the same product—the European Court of Justice ruled against this pricing practice, requiring insurance companies to charge both genders the same amount. In the U.S., Obamacare requires health insurers to charge both genders equally. There are women on both sides of this argument, debating what fair and equitable pricing means.

Gender price discrimination is illegal in many states but it can be quite tricky to determine when two products are really the same and when they are different. For example, Miami-Dade County has ordinances that prohibit gender pricing for dry cleaning. The gray area is this: "A business is permitted to charge a different price if the goods or services involve more time, difficulty or cost. In other words, consideration must be given to the quality and complexity of the goods or services to determine whether or not you have been discriminated against."

New York City’s Department of Consumer Affairs (DCA) has stricter ordinances. Inherently biased gender terms, such as shirts and blouses, are not enough—prices can differentiate for shirts with and without ruffles, but not between shirts and blouses. In recent years, the New York DCA has cracked down on gender pricing violations in the city. Inspectors patrol businesses regularly, and in 2012 they handed out 361 notices. Last year, there were 195 violations, and this year’s violation count is already over 200.

Economist and Yale Law School professor Ian Ayres published a landmark study in 1991 showing that Caucasian woman are charged a 40 percent markup compared to Caucasian men at new-car dealerships. Ayres says that as of 2000, there's 43 states that prohibit gendered price discrimination in public accommodations.

"There’s no general federal law prohibiting price discrimination on the basis of gender," says Ayres. "There's the Unruh Act in California which is a matter of state law. There’s an increasing number of states and municipalities that have prohibited gender price discrimination in public accommodations."

So do women always pay more? Not always. Often, nightclubs charge women less for entry, a practice California has banned. Similarly, nail salons often charge men more than women, reportedly because so-called man-icures require more work.

At least a couple men have spoken out against ladies' night: GWU law professor John Banzhaf, and New York lawyer Roy Den Hollander (who has been rightly ridiculed for his comments regarding why he sued: he hates feminists).