Vor 60 Jahren hätte niemand das Wort ,,Sex" laut ausgesprochen. Sex war privat, aber selbst
da versteckt und lustbefreit.
You can build a number of words adding "-befreit" (literally: "free of"). So it's not really a neologism, it's a construction that's not uncommon. But like many compound words, a dictionary won't list those.
It's also not uncommon that this construction is used tongue-in-cheek style, so you have to be careful when interpreting it. Here, according to the context, the meaning is just the plain one: Sex was "officially" just seen as a duty ("eheliche Pflicht") between a married couple, and the whole aspect that it can be fun was (officially) completely ignored. So even if people enjoyed it, no one would have admitted so it in public.