Adultery used to be the main (and close to the only) ground for a divorce in the past. Not much evidence was needed only 60 years or so ago. This might be because Judges and lawyers were too embarrassed to ask that awful question of a lady, “have you had sexual intercourse with a man other than your husband during the marriage, other than by rape?” That is the plain English definition of adultery now.
Note that adultery is not just sexual contact alone and is not sexual conduct with the same sex. (That might be a different ground of “unreasonable behaviour“)
But things are improving! A case in 1948 (Barnacle) showed, to us now, some startling views of adultery. This is an outline extract:
Wife said she understood adultery to mean having a child by someone else. Accepting this evidence and allowing the decree to stand, the judge said he had personally met otherwise well-educated men who thought it was not adultery if the woman was over 50, and the King’s Proctor had come across people who thought it was not adultery during the daytime.