A reason not being given much attention (at least as far as the UK went) was that people had an awful lot less sex.
There are various, really quite obvious reasons for this, such as:
- Poor living conditions. If you lived during the Industrial Revolution you were probably sharing a room with 3 or 4 other people; even more if you were in the slums, so you would have no privacy. You might be sleeping on the floor, and your house might be inadequately heated. Human Documents of the Industrial Revolution by E Royston Pike is a very interesting (and sobering) collection of contemporary descriptions of UK towns in the nineteenth century.
If you were too poor to support yourself, you entered the workhouse, where the sexes were segregated.
- Poor health. Compared to now, people had poorer quality food and less of it, and therefore, I assumes, less libido. Also people would have simply endured various health conditions easily cured now. I doubt if you suffered from kidney disease or lumbago that sex would be on your mind much.
- Fear of pregnancy. Apart from the obvious health risks, pregnancy outside marriage was a disaster for women.
- Disapproval of sexuality. There is an idea that sexuality can't be contained, and therefore people in the past didn't contain it. Actually, I think they did. Until recent times, celibacy was quite normal - not just in Catholic countries - in fact it was the expected state if you were not married. Controlling your sexual appetites was considered healthy, in the same way that we consider moderating or eliminating alcohol or other various things healthy. Until very recent times, there was lots of peer pressure away from sex and none towards it.
Diarmaid Ferriter has done some very interesting research into sexuality in Ireland and how it was controlled. Ireland's probably an extreme example, because it was anything but a permissive society until very recent times, but what's really striking is how references to any any discussion of sex was basically eliminated from public space: sex was an entirely private thing that belonged to marriage only.
I have never noticed is alternatives to vaginal intercourse being discussed. I read one account of some kirk session in Scotland that dealt with a local man who "knew" a woman "in the forbidden way". I assume this means anal, and clearly its description as "the forbidden way" meant it was a known thing. Masturbation, however, was always frowned upon and oral doesn't turn up in anything I've read as far as I remember. I'm not surprised: people's private parts must have stunk to high heaven. I suspect that insofar that alternatives to PIV were going on, it was probably anal but nothing else.
There is literature suggesting some promiscuity, e.g. Harris' List of Covent Garden Ladies, and the frightful Secret History, but in general it concerns men who were rich enough to pay for these things and therefore aren't representative.