If a "fair" system is one where gender-balancing is the goal, then one solution would be to create a differential between male / female prices, to discourage women / encourage men.
So if you had (say) 60 women and 40 men, and normally charged £5 entry, you could charge the women £6 and the men £4. Same amount of revenue, but in theory that'd encourage parity of numbers - at which point you could return the pricing to equal.
(Same sort of thing that nightclubs do with "Ladies' nights", but in reverse)
Of course, such a scheme would be both impractical and vastly disruptive, you'd probably find yourself with 0 men and 0 women in short order if you tried that...
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