Gee. Between 88% of their men killed in the war, and the millions of their own people they killed...who would be left for fighting the enemy, killing more of their own people, or even procreating with women?
Not 88% total men...just 88% born during a certain year, 1922. But it is estimated 27-30 million Russians died during the war, the great majority male. That left a lot missing fathers for the next generation.