Family

Family is a group of people related by birth or marriage. Marx and Engels saw the rise of the "traditional family" as bound up with the development of property relations. Engels goes into detail about this in his The Origin of the Family, Private Property and the State:

Monogamy arose from the concentration of considerable wealth in the hands of a single individual — a man — and from the need to bequeath this wealth to the children of that man and of no other. For this purpose, the monogamy of the woman was required, not that of the man, so this monogamy of the woman did not in any way interfere with open or concealed polygamy on the part of the man. But by transforming by far the greater portion, at any rate, of permanent, heritable wealth — the means of production — into social property, the coming social revolution will reduce to a minimum all this anxiety about bequeathing and inheriting. Having arisen from economic causes, will monogamy then disappear when these causes disappear?

One might answer, not without reason: far from disappearing, it will, on the contrary, be realized completely. For with the transformation of the means of production into social property there will disappear also wage-labor, the proletariat, and therefore the necessity for a certain — statistically calculable — number of women to surrender themselves for money. Prostitution disappears; monogamy, instead of collapsing, at last becomes a reality — also for men.

In any case, therefore, the position of men will be very much altered. But the position of women, of all women, also undergoes significant change. With the transfer of the means of production into common ownership, the single family ceases to be the economic unit of society. Private housekeeping is transformed into a social industry. The care and education of the children becomes a public affair; society looks after all children alike, whether they are legitimate or not. This removes all the anxiety about the “consequences,” which today is the most essential social — moral as well as economic — factor that prevents a girl from giving herself completely to the man she loves.

Engels then goes on to note how traditionally, marriage was not about love. Only relatively recently had the concept of love started to enter discussion of marriage itself:

And as sexual love is by its nature exclusive — although at present this exclusiveness is fully realized only in the woman — the marriage based on sexual love is by its nature individual marriage. . . If now the economic considerations also disappear which made women put up with the habitual infidelity of their husbands — concern for their own means of existence and still more for their children’s future — then, according to all previous experience, the equality of woman thereby achieved will tend infinitely more to make men really monogamous than to make women polyandrous.

In other words, Engels assumes that if anything, the abolition of the "traditional family" will strengthen monogamy by creating equal relations among husband and wife and having their decision to marry be based on nothing but love. But if that does not pan out, and monogamy is indeed destined to become extinct (just as earlier forms of the family went extinct), then Engels' conclusion remains:

What we can now conjecture about the way in which sexual relations will be ordered after the impending overthrow of capitalist production is mainly of a negative character, limited for the most part to what will disappear. But what will there be new? That will be answered when a new generation has grown up: a generation of men who never in their lives have known what it is to buy a woman’s surrender with money or any other social instrument of power; a generation of women who have never known what it is to give themselves to a man from any other considerations than real love, or to refuse to give themselves to their lover from fear of the economic consequences. When these people are in the world, they will care precious little what anybody today thinks they ought to do; they will make their own practice and their corresponding public opinion about the practice of each individual — and that will be the end of it.