I still remember the first time I got a hard on while reading a BOOK.
I was about 12 years old and was reading a science fiction short story where the entire world was bi sexual and pretty much hooked up whenever and wherever they wanted. At least that is how I remember it. It was a huge eye opener.
I have tried to track down the book again. It was probably a Robert Heinlein or Arthur Clarke, but I have not but have not been able to find it.
More like later Heinlein ,could be anything from Stranger in a Strange Land onwards, though it sounds more like Time Enough for Love (or one of the sequels).
Doesn't sound like Clarke , don't remember much sex in his books.