This reminded me of an IMDB message-board comment on the same film:
I just watched the film for the first time, which I very much enjoyed, but had thoughts similar to yours, specifically that the portrayal of a sort of rampant, open sexuality was more aligned with the publication date of the novel (1965) than the story's WWII setting. The group scene with the nurses and the German soldiers seemed especially out of place. I thought perhaps the apparent shift in sexual mores was a response to the ongoing war, a sort of "eat and drink, for tomorrow we die."
This argument might make sense if the author of the novel had been some trendy young twentysomething trying to make a literary splash in the mid-Sixties, but in fact Bohumil Hrabal was born in 1914 and was already pushing thirty during the period in which his (partly autobiographical) novel is set. And you'll also find similarly ribald depictions of the behaviour of young Czechs in the first half of the twentieth century in the work of plenty of other novelists - Josef Skvorecky and Jaroslav Hasek both spring to mind.
The problem as I see it isn't imposing "1960s liberal ideas" on wartime Czechoslovakia, it's imposing American notions of "acceptable" behaviour in the 1940s onto what was in many respects a very different society.