I'm with GohanLSSJ2 here. As soon as people like Alexandra Kollontai (an early proponent of free love) were sidelined after 1917, the original Politburo grew more and more socially conservative. Stalin did indeed ban abortion later on. How should a proletarian society grow if there are not enough kids to be raised in a Soviet style?
The magic trick word here is "dialectic".
That reminds of a passage from Arthur Koestler in the book "The God That Failed", where he describes his brief involvement in the German KPD and his subsequent disillusion with communism. I only have the German edition, so the translation is by myself:
"Bourgeois morality was objectionable, but Free Love was objectionable as well, so the only true, correct, concrete position on the sex drive was the "proletarian morality". This consisted of getting married in a nice way, being faithful to the spouse, and bearing as many proletarian children as possible. But wasn't this the same like the bourgeois morality? - This question, comrade, shows that you are thinking in mechanistic and not dialectical terms. What is the difference between a gun in the hands of a policeman and a gun in the hands of a member of the revolutionary working class? The difference between a gun in the hands of a policeman and a gun in the hands of a member of the revolutionary working class is that the policeman is a lackey of the ruling class, and thus his gun is a tool of supression, whereas the same gun in the hands of a revolutionary is a means to the liberation of the supressed masses. The same goes for the difference between the so-called "bourgeois" morality and the proletarian morality. The institution of marriage, which only mirrors the decay of bourgeois morality in a capitalist society, is transformed through a dialectical, functional change in a healthy, proletarian society. Did you get that, comrade, or shall I repeat my answer in a more concrete way?"