Anna was a little dim

Karenin is more than generous and understanding with her. Perhaps because of the age difference he may have thought that she might stray and decided to let her have what she wanted without kicking up too much of a fuss.

He was willing to ignore the affair as long as he was not embarrassed and he was willing to raise the child (!) as his own as long as Vronsky never set foot in his house. How many women in her situation would have loved to have that option. I don't blame him for losing his temper and attempting to divorce her after seeing Vronsky enter his home. If Karenin had had an affair I think he would have had enough respect not to bring his mistress home to the house he shared with his wife.

Because of her lust and silliness I cannot say that I am sorry that she lost her mind although it was very hypocritical that Vronsky was welcome at his friend's party but Anna was not.