I was 22 in 1986 and as a male of that age I was mainly interested in music, cars, making money, and women. I remember the "moral" climate very well indeed. When I left school at 18 - not before - I acquired a girlfriend from among my social circle, which was much like SJL's although a bit more focused on university. Like my mates, I was disappointed but not entirely surprised to discover that she wouldn't sleep with me because she "didn't believe in it". This was commonplace - at that time you were actually quite fortunate to find a woman who would sleep with you.
There were exceptions; I knew a woman of my own age who had slept with 500 men by the time I last touch with here about 2000. But I recall going to parties, chatting with women and being taken aside by mates who would warn me that "she doesn't do the business". It didn't change at university; it was noted that women finally gave it up, if they did, in their fourth term, when they observed how young the freshers seemed to be. As late as the early 90s I had a 25-year-old girlfriend who wouldn't do this. Another girlfriend, who lived with her parents but stayed with me at weekends in my 1-bedroom flat, related that her mother thought we didn't sleep together because "he probably doesn't want to get into all that".
This was essentially a pre-1960s attitude to pre-marital sex that SJL's parents would have shared, and that they probably assumed SJL also shared. They probably liked to imagine she didn't sleep with her boyfriends, and she would certainly have managed carefully what they did know.
As we speculated previously, however, it could easily be that SJL had slept with 50 men so far that year, and several hundred in total (or something). This would only be one new one a week since the age of 20. Maybe she slept with women too. Given her looks, she could easily have managed this. She would have to have been nonchalant about her personal safety and perhaps prepared to sleep with married or spoken-for men to scratch her itch. Perhaps that's what in her diaries.
Any of this would have appalled her parents (actually, if she had slept with 10, they would probably have been appalled by that too). It seems clear that they imagined this to be shameful and wanted it suppressed in the interests of managing her reputation.
More pertinently, given that about 1% of people are psychopaths, she may have slept with two or three, and perhaps one or more took violent exception to the other strings to her bow. The wife of a married lover could have taken exception to her in a violent way.
Her parents' insistence on managing the public perception of SJL to protect her posthumous reputation thus did her a huge disservice. Whatever you think happened that day, it seems clear that she was taken and killed by someone she knew. Hence the focus needed to be on who she knew and what was afoot between her and that person. That this was not done properly has ensured that her killer got away with it.