"Genuinely desirable" seems like the problem here, in that it's conflating base sexual attraction with a more pragmatic evaluation of someone's prospects.

Beta males certainly have many admirable qualities; they're reliable productive and civil, usually friendly and loyal as well. But those qualities, while again being very important, are simply not attractive.

Alpha males, on the other hand, are really quite a menace. The Dark Triad traits which make them attractive also mean they are shiftless and poor contributors to society, at least for the most part.

Hence the pattern of "Alpha fucks, Beta bucks." Women want to get the Alpha but will, if forced to by circumstances, trade sex to Betas for resources / security.

In that context, female "Betas" would be the low-risk women men settle for reluctantly while "Alphas" would be high-risk women who are highly sought after.