It seems it's women that underestimate male attractiveness.

What would that even mean? Remember that attractiveness is a two-place word. Women are underestimating how attractive men are to whom? Would a more natural description of the OKC data that men are in average less attractive to women than vice versa?

(I think you misunderstood what MugaSofer meant, which he better explained in his reply. IIUC what he hypothesized is that if you picked an actually median women and you asked people what fraction of the female population are less attractive than her, you'd get an answer much less than 50% -- e.g. because below-median women are underrepresented in mass media compared to above-median ones, or something.)

(obvious confounds: people that use okcupid may not be representative of the population generally, both for the raters and ratees.)

Well, for starters, it's mainly used by single people, so very desirable people are filtered out unless they are also very picky.