Tomas had been married for less than two years when he divorced his wife, and while he initially fought for custody of his infant son, Simon, he quickly decided not to see him anymore either. Tomas's parents were furious with his decision to abandon his son, so Tomas quit seeing his parents as well. Since then, his life has been a series of sexual relationships with women.
Tomas abandons his entire family as a means to achieve lightness. Marriage and fatherhood are incredibly serious—and therefore heavy—roles, as is Tomas's relationship with his own parents, so he leaves them all behind. Instead, Tomas's life is full of meaningless sex with hundreds of women, a much lighter existence compared to his previous life as a father and a son.
Tomas abides by a set of rules regarding his mistresses. He either sees a woman three times, back to back, or he sees a woman for years and separates each meeting by at least three weeks. Not every woman Tomas meets appreciates his approach to relationships, but Sabina does. Sabina understands Tomas, and she likes him because he is "the complete opposite of kitsch."
Sabina lives her whole life trying to avoid kitsch, an aesthetic ideal that Kundera extends to life in general. In Sabina's opinion, traditional marriage and family are examples of kitsch, and Tomas is certainly the anti-family—and so "the complete opposite of kitsch."