Natalia Goncharova did come from a noble family, although they were very poor by Western standards, like a lot of the Russian nobility. Her father, as I recall, was either an alcoholic or a madman or both (he attacked his wife once with a knife, and was then kept locked up in a set of rooms separate from the rest of the family). Natalia did not have a dowry, and this is probably why she ended up marrying Pushkin, who was not considered a good catch. On the contrary - he also lacked a personal fortune and had a bad reputation as a gambler and ladies' man (although he was also a great poet, this did not count for very much in the eyes of the aristocracy). With her astonishing beauty, it surprised society that Natalia did not make a better match. But Pushkin was deeply in love with her and remained so for the rest of his life, calling her "my angel" and writing many famous poems about her.
Natalia's own reputation has suffered greatly since Pushkin's death - in some quarters she is still blamed for it - and she is still the subject of great controversy amongst Russian "Pushkinists," although not so much amongst Western ones, if I have read the sources correctly (it's quite possible I have not). Nicholas I's chief Western biographer, W. Bruce Lincoln, states that the tsar was only unfaithful to his wife after thirty years of marriage, and then only with a lady-in-waiting named Varvara Nelidova. No mention is made of Nicholas ever taking as his mistress the widow of Pushkin. As far as I know, this relationship only ever existed as a matter of rumor and gossip, chiefly amongst Nicholas' many enemies.