The man you do this for has to be more than just an average man or else why make this sacrifice? Why not just marry an average man and have him to yourself?
Megan, you are a tremendous and amazing high-value woman hampered only by autism and a bit of previously-lived trauma. You are thus in the far-above-rubies category, but the answer to your question is that (a) the majority of unmarried women aren't even average, and (b) there are significantly more unmarried women seeking marriage than there are unmarried men seeking marriage, and thus (c) the average unmarried woman doesn't come close to having the luxury to marry an
average man and have him all to herself. Especially given the continued stigma about plural marriage, yes, in a partial sense, you're correct that,
if she's willing to share a husband, the average unmarried woman has the leverage to expect a better class of man than she would otherwise acquire -- but even with that stigma firmly in place the average unmarried woman is light years away from being able to demand a significantly-better-than-average man if she's willing to share him, and this will only become exacerbated as the stigma abates --
which it will. No one can force women to accept the level of men they deserve, but none of that will stop the women who hold out for higher than they deserve from dying alone in poverty with their cats and dogs . . . and rats and squirrels.
Both sides of the gender line are in need of acquiring a much more realistic outlook about their true Sexual/Mating/Marriage Marketplace Values. Among men, this is most especially true among those of us seeking plural marriage in the context of the overwhelming societal opposition that still exists (my advice to me is to be honest about what one's SMV would be if one were single,
and then cut that number at least in half), but, among women, delusions related to failure to face up to SMV are close to universal, plural marriage or no plural marriage.
Generally speaking, an unmarried woman simply doesn't have a leg to stand on when she begins to expect that she deserves a high-value man just because she's willing to 'settle' for polygyny. Delusion (bordering on mental illness) is rampant in our predominantly progressive feminist culture these days, though, so it's so much the norm that it appears to be righteous, but that doesn't make it so.