The debate is far more nuanced than which version of the sport is more popular. Prize money in tennis is a reward but also - like the gold medal in Olympics - a symbol for victory. If it's merely a function of popularity then Federer should earn more for his Grand Slam win than someone like Murray - that'd be ludicrous.
On every metric, men's tennis is undoubtedly superior but there are couple of gender-specific reasons for this. First, biologically men will always be stronger and faster; we cannot hold this against women. In Olympic boxing, both featherweight and heavyweight champions get the same gold (sadly, in professional boxing prize money is now almost entirely a function of popularity ). If we consider prize money as a symbol then equal pay is entirely justified.
Second, women across the world have found it difficult to access sport. The much smaller talent pool explains some of the inferiority. Equal pay is one way to compensate for inherently sexist societies which hinder women's tennis development. In such an unequal world, I think sport should take the lead even if it may seem unfair on principles of meritocracy.