If women are systematically underpaid at a company, that's strong grounds for a lawsuit. Same for Blacks, or gays, or older people, etc..
But what about ugly people? If a company systematically discriminates against ugly people (either through not employing them or paying them less), is that grounds for a lawsuit?
Slightly more severely, what if the attractiveness-discrimination was based on sexuality? I.e. recruiters sexually attracted to women were more likely to discriminate against unattractive women, but by the same token, recruiters sexually attracted to men were more likely to discriminate against unattractive men. Technically, the discrimination isn't really sex-based (since it's applied to both sexes), but attractiveness-based.
Assume that the evidence is overwhelmingly substantial (e.g. 200 people apply for the company, 100 of them are hired, and an independent third-party ranked the 200 people based on attractiveness, and the top-100 attractive people turn out to be the ones that were hired). And assume that attractiveness has nothing to do with their job (e.g. it's a tech company). And if absolutely necessary, you can even assume that the recruiters explicitly admit to discriminating based off attractiveness.
Lastly, on a related note, let's say unattractiveness isn't a protected class (which I imagine will be the case). However, disability is a protected class. At some point, can a condition that makes one unattractive be legally construed as a "disability" which protects one from discrimination? For example, a cleft lip, Vitiligo, facial paralysis, a lazy eye, burn scars on ones face, etc..