Friedrich Engels
co-author of The Communist Manifesto

"...the first condition for the liberation of the wife is to bring the whole female sex back into public industry, and that this in turn demands the abolition of the monogamous family as the economic unit of society."

Origins of the Family, Private Property, and the State
II. The Family
4. The Monogamous Family

https://www.marxists.org/archive/marx/works/1884/origin-family/ch02d.htm