All sorts of rumour on this one – that he was an ascetic, that the relationship was purely convenient for party purposes, that he was gay, etc.
But when Nadya arrived in Shushenskoye in 1898, they set about swimming, ice-skating, hiking and … energetic bonking, as randy young couples do. So why didn’t she become pregnant? In reply to the discreet inquiries from Lenin’s mother, Maria Alexandrovna, as to when she might expect a grandchild, Nadya wrote:
As far as my health is concerned I am quite well but as far as concerns the arrival of a little bird – there the situation is, unfortunately, bad; somehow no little bird wants to come.
Krupskaya in Lenin’s Collected Works, Volume 37, p. 578.