It's just perceived selection pool, mostly consisting of guys they despise and use for feeling better about themselves and desirable (they will tell you openly once they are comfortable with you they would prefer those guys not to exist at all), or momentarily top guys that are difficult to reach even for the prettiest/smartest of them, and where there is a literally brutal competition between them.

If you are one of those top guys, you have to invent elaborate schemes how to reject a female so that she thinks it was her decision and leaves you alone ("how much nice/beta do I have to go until I start repulsing her?"), or just state/enforce expectations you have with them, maybe being shocked by mind-blowing admission that they would be willing to share you with other women only if you kept them as well.

The problem of this is that it leaves females that are used to one-time attention from a top guy thinking that's their attractiveness level for the rest of their life and anyone marrying them walks straight into hell of "being never enough". So overall for society enforced monogamy might be way better than what we have now, i.e. most females flocking to top guys and using the rest for money, validation or easy life until their attractiveness fades away and they become broken shelves of persons.

But hey, they front-loaded their life, trading exciting time in their prime for desolation of middle-to-older age. Just please don't force men to support them with taxes once they fall through the bottom. Those men didn't study/work hard to trade their prime years for supporting women that didn't give them time of day when they still were interested in them. That's not fair either; no wonder many of those men go for pathetic sex dolls and VR porn, or just leave society completely.