At first, the hitch-hiking game is a lot of fun for both the young man and his girlfriend. The young man likes to play the part of the stranger picking up a sexually-available young woman by the side of the road. It feeds into his toxic sense of masculinity, with its overriding need to see women purely as sex objects to be hunted down and seduced.
As for the girlfriend, she enjoys the game because it gives her a sense of power and control. By putting on the role of a sexually-liberated woman, she feels more confident in her own body, an entirely new experience for this normally self-conscious, embarrassed young lady.
For good measure, the young woman finds that, in her new role, she doesn't feel as jealous towards other women as she used to. At the same time, she enjoys the feeling that she's pleasing her boyfriend by behaving the way she does.
In due course, however, the young man begins to resent his girlfriend's behavior. To him, she's behaving just like all the many other women he's bedded over the years. And as this inveterate womanizer didn't have any respect for them, so he has none for his girlfriend.
It would seem, then, that in the patriarchal society depicted by Kundera, adopting a different personal or social identity is purely a male privilege. Men can act out different roles without incurring any serious censure from society. But for women it's a different story. They're forced to pick a side in the age-old virgin/whore dichotomy and stick to it.