These were on during a very impressionable time of my life.
1) Ones with Shannon Tweed (Mrs Gene Simmonds) were invariably better than those without.
2) They generally came in two varieties: Erotic dramas, and erotic thrillers. Dramas were usually better, and would typically involve a bored housewife exploring her sexuality. Thrillers would normally involve a cop trying to track down a weirdo serial killer, and becoming emotionally involved with the victim's sister.
3) Occasionally, Channel 5 would throw a curve ball and put on a Russ Meyer film (pretty good), or a 1970's British sex-com like Confessions of a Window Cleaner. These were not so good, though were interesting for those of us interested in the development of British cinema.
4) You could often work out from the number of women mentioned in the opening credits if there was a decent chance of a lesbian scene.
5) Because my bedroom was right above the living room, I had to watch all these on an incredibly low volume. I subsequently became a fairly adept lip-reader.