Engels reads monogamy as a sign of the coming of civilization, the end of barbarism, and the crystallization of certain romantique ideologies whose influence may extend as far back as antiquity. As a way of organizing kinship, monogamy was introduced primarily by husbands who wanted undisputed heirs to their wealth and property. He believes there are natural and biological tendencies which undercut it, i.e. natural selection, which narrows the large circle of sexual partners until there remains only one heterosexual, monogamous, pair. He rightly criticizes the moralizing interpretation of his contemporaries, who cannot see the strict order and laws of the non-monogamous sexual practices of earlier times.
Yet Engels himself is not without his own moralizing tendencies, condemning sodomy as an aberration. He does not gesture toward a transformation of monogamy as he knew it, even with the overthrow of capitalism. Instead he only suggests a newfound freedom for women, whose sexualities have been violently regulated by that of their male partners. I see something Foucault may take influence from. We cannot know those future forms and practices in advance. Engels would agree, I think.