Worn,
I don't know of many men who wouldn't want multiple wives
.
Although it's likely that wealthy men would enjoy the most competition from women because of their market value, women would still be able to freely participate in the marketplace. Unlike many polygamous societies among religious communities and throughout history, the legal system would prevent women from being bartered and sold by the man who "owned her." Women would be guaranteed the legal right to negotiate, terminate, and choose the contracts they entered.
Secondly, legal protections would provide women with options to avoid an exploitative contract. First, the right to negotiate, enter and terminate would allow a woman to negotiate some kind of insurance and/or exit if the arrangement did not suit her needs. Additionally, there would be legal protections against certain kinds of exploitative contractual conditions, as there are today, to prevent a party from being taken advantage of due to hardship or mental disability (temporary or not). Second, the legal right to participate in the economy would allow women to invest in their own careers and assets, providing them with the freedom to exit a bad contract. Third, because many men would lose out, assuming that this system would have a tendency to favor polygamy, women would also receive a lot of competition. Four, polygamy would not be the only possible arrangement. Parties could still enter monogamous contracts, and I think a polyamorous arrangement including multiple men and women in the same marriage might become popular. So while I agree with you that wealthy men would likely have more market power, I think you are exaggerating the extent women might be exploited by neglecting many legal and market conditions.
Now, to your question about reproduction and making marriage contracts legally binding. With respect to reproduction, it seems your question has more to do with finances. Moreover, it also seems you are assuming that men would necessarily be the sole income earner. A man earning $200,000 USD as a small business owner may be in a situation you described, but it would be in the interest of all parties if some of the wives worked. For example, the living standard of two couples earning $50,000 improves (assuming no financial infidelity) if they pool their resources. Since women can participate in the economy, marriages with more than two parties would likely have more than one member working. This could be beneficial too, as your hypothetical family could have 1 working man, 2 working women, and 1 woman who stays home to care for the children. If you think about it, that's more efficient than the nuclear, monogamous family. Additionally, it is better for the environment. With respect to making marriage contracts legally binding, I don't see why this is a problem. The government can treat marriages just like it does any other business contract.