The Unbearable Lightness of Being
How we cite our quotes:
The brain appears to possess a special area which we might call poetic memory and which records everything that charms or touches us, that makes our lives beautiful. From the time he met Tereza, no woman had the right to leave the slightest impression on that part of his brain. (5.12.6)
So Tomas is faithful, in an important way, to Tereza. It's just that his brand of fidelity is an unusual one.