I agree with your view on prostitution. Here is a somewhat longer analysis, from this document (page 17, in Spanish).

Prostitution: is the phenomenon of sexual exploitation of women’s bodies, being absolutely the fruit of historical material conditions such as private property, inheritance and monogamy. The system’s control of the utilization of women’s bodies and sexuality implies a total patriarchal and capitalist domination of sexuality, which finds its climax in the buying and selling of sex and consent. In this way, the market makes a niche for itself at the expense of the sexual exploitation of women, deepening their objectification and sexualization as a dehumanized subject, devoid of the basic rights of society. We can analyze prostitution as the greatest form of patriarchal and capitalist exploitation of working women. Prostitution also applies the commercial logic to sexual activity, under the premise that “everything that can be paid for can be sold”. Prostitution is not work, since it does not use labor power, but turns women into a means of production, i.e., they are stripped of their human category to become a commodity.

Regarding polyamoury, in my view that is mainly a phenomenon in the United States. It could be related to the fact individualist ideology is so widespread there, that people arent able to compromise enough with a potential partner to form a stable relationship.