There is a fascinating graphic, Sex by Numbers on the Wellcome Collection website (yet another spinoff from the Institute of Sexology exhibition).
A lot of these figures are intriguing, but for so many of them we don't have a baseline, or at least, not a baseline lying further back than the original National Survey of Sexual Attitudes and Lifestyle published in 1994. The generational changes, at least among women, do continue a trend that was already becoming apparent when Eustace Chesser undertook his methodologically somewhat idiosyncratic but nonetheless suggestive study of The Sexual, Marital, and Family Relationships of the Englishwoman (1956) - the women born in the 1930s who have had fewer partners than those in the 1970s were those who, in the 1950s, married earlier and had higher expectations of sexual satisfaction than their foremothers.
Similarly the falling age of first intercourse can be mapped against other early surveys such as Geoffrey Gorer's Exploring English Character (1955) and Michael Schofield's The Sexual Behaviour of Young People (1965) to show a continuing trend over time.
However, some of the other figures cannot be set in a similar context, and I think one might be hesitant to suggest that they represent e.g. ongoing decline in the frequency of intercourse to a point at which nobody has sex at all (though I have come across dystopian sf that posits this exactly - either the drive has vanished or the dystopic State forbids with extreme penalty - an early instance would be E M Forster's The Machine Stops). As this (apparent) decline is set in a context of people living their lives to a greater extent online, maybe what should be being prophesied as the outcome ought to be sophisticated teledildonics?
The lowered toleration for infidelity may have something to do with the removal of the legal difficulties and social stigma around divorce (still very much present at the time Gorer and Chesser were writing) which kept people in marriages they might have preferred to exit, not to mention the (related) rise in the acceptability of cohabitation.
Some of the statistics lead me to wonder about how far they might reflect longer patterns of behaviour - e.g. what percentage of men in the population at a given period were actually paying for sex (and whether it was always a good deal less than All Men) and whether this was more likely to happen 'playing away' - or whether they are idiosyncratic blips.