The word "idyll" is important to Tereza, and the narrator stops to consider why.
In light of the Old Testament, he argues, an idyll is an image of Paradise that remains with us. In such an image, time runs circularly, a repeated routine.
When people live in the country, life moves this way, circularly, and so they maintain this glimmer of Paradise. That is why the countryside conjures an image of an idyll for Teresa.
Adam, if he saw his reflection, did not recognize that he was seeing himself. This makes him like Karenin, and unlike Tereza who has spent so much time trying to find herself by staring into the mirror.
Because Adam was like Karenin, it means that he was not like man.
Man's longing for Paradise, argues the narrator, is man's longing not to be man.
When Tereza was a child, she was always disgusted by her mother's menstrual period, and wished that her mother had the shame to hide it.
But don't forget that, despite the masculine name, Karenin is female, and so has periods. Interestingly, Tereza finds to be amusing, not disgusting. Why is this so? Because animals have no concept of disgust. This is why Tereza feels so lighthearted with him.
It occurs to Tereza that her love for Karenin is better than her love for Tomas, because it is a selfless love.
She didn't expect anything back from Karenin, she never tried to change him, and her love for him was voluntary.
Animals, adds the narrator, were not expelled from Paradise, which means that the love for an animal is idyllic.
Additionally, Karenin was happy to live his life in a circular manner, the same routine every day. He never got bored, whereas man needs repetition.
Karenin could be happy, because "happiness is the longing for repetition" (7.4.13).