I am not sure how well Heinlein's polyandrous Loonies[1] would match real-life scenarios. This did not happen in New South Wales where male convicts outnumbered the female convicts six to one[2].
Males and females are not symmetrical. A female who never reproduces because she wants to be faithful to a mate who doesn't exist is giving up 100% of her reproduction. A female who stays faithful to a mate who does exist is giving up 0%, in terms of quantity -- one mate can provide all the children a female is capable of having.
[1] https://en.m.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress [2] https://en.m.wikipedia.org/wiki/Convict_women_in_Australia