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It's all a matter of negociation and agreement.
And in the era of DNA tests there can not be a fraud which the children anymore. The breeding partner will have to be well considered.
There is no symmetry between men and women as the biological starting points are not symmetrical. A man can have 10 women and impregnate them all and a woman can not do anything resembling, no matter how free she is. This is why a productive "harem" structure can only be a man with numerous women and not a woman with numerous men.
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