I believe simple economics (demand and supply) is also at play here. Clubs have operating costs to cover, so on a typical night depending on numbers they know on average how much per head to charge to cover costs. But here's the snag:
1. Most know typically they can't charge per head as many ladies don't see why they should pay the same as guys. Unfair, but it's human interaction for you: call it "men picking up the bill". So they charge men more.
2. Clubs know the presence of the average single girl can get maybe 2 couples and 3 interested guys in the venue (modest numbers). Clubs just need to incentivise more girls be they straight, bi or gay.. seeking men, women it couples.
3. Combine the two and the average cost per head is tweaked: lower the cost for girls, bump the cost for guys and everything works out subsidised after profit.
Is it fair? Probably not, but that's economics. If some law demanded the pricing be made equal per head, it could kill the club scene.