Well then, you're not placing this in a real world context.
There is not a magic top level of small bands who are all male. There are 100s of bands that have the exact same talent that Lyman could choose to play smaller stages, because there is a bias in alt music (and the world at large) an all male band is likely to be chosen above a female band, despite them having the same skill level - this happens in pretty much all walks of life, our scene is the one we can impact.
Regardless of what criteria Lyman uses to book bands, I honestly doubt booking more men than woman is a conscious effort. Even if it works out that more pop/punk fans prefer guy singers than girl singers, and that's why (indirectly) more male acts get booked, that is an issue far beyond Warped. And honestly, we may not want to face it, but that may be the truth. If that opinion is based on gender, then that's clearly wrong and sexist and an issue in it's own, but if that opinion is truly based on personal preference, then that's just how the scene is.
I'll be honest with you, I listen to a lot of girl singers and girl bands, but not many that are in the "scene." I love girls singing indie, acoustic, pop, etc (Tegan and Sara, Rilo Kiley, Broods, plenty of other indie bands..) but I prefer guy vocalists in heavier music. Does that make me sexist? No. It means I'm able to have an opinion. Maybe it just works out that more people prefer men in the scene than women, which in that case is evident when festivals are being booked, because there are not as many popular or appealing female bands to the public.
Again, this is a hypothetical situation and in no way am I claiming that is the case. But because you said "because there is a bias in alt music (and the world at large) an all male band is likely to be chosen above a female band, despite them having the same skill level," that doesn't' mean it's a for a sexist reason. There's just not as many female bands in the scene, and maybe there's not as many being booked because there's not such a high demand for them. It may have nothing to do with the fact that they're women and not men in a sexist sense.