You need to read Makarenko's lecture on sex ed on this to better understand, I will try to summarize: this is a classless society but the freedom gained after the revolution led to "free love" way and

Such sex life unfailingly coarsens human relationships, vulgarizing them and leading to disintegration of the individual, to unhappiness, to the destruction of the family and orphaning of children

Instead:

In his relations with women or men, a Soviet man may not ignore the requirements of social morality which always stand guard over the interests of the whole society. In the sphere of sex, this social morality makes definite demands on every citizen. Parents must bring up the children so that they will become people whose behavior does not conflict with social morality.

Sex according to Soviet morales must happen only if there is love and family and order.