Tomas is incapable of giving up his erotic friendships and being faithful to Tereza. He also sees no reason to do so, because he doesn't think that they threaten his love for her.
And yet, when he goes out to meet other women, he feels it is distasteful. He can't even bring himself to cheat on her unless he drinks alcohol first.
Unfortunately, Tereza then knows he's cheated every time she smells alcohol on his breath.
Sabina still makes Tomas happy, because he knows she is discreet, and she is a symbol of his bachelor past.
One day, while Tomas and Sabina are having sex, she catches him looking at his watch (he's obviously worried about getting home in time for Tereza).
Afterwards, as he gets dressed, he cannot find his sock.
Sabina lazily tells him that he's becoming the focus of her paintings – the meeting of two different worlds. In Tomas's case, he is part Libertine and lover, and part Tristan, always thinking of his Tereza.
Tomas never finds his sock. He knows that Sabina hid it to get back at him for looking at his watch during sex, so that he would have to go home one sock short.
He realizes that Sabina resents his love for Tereza as much as Tereza resents his lust for Sabina.