It used to be far easier to define in the Old Days, when it was the norm to be married, which, among its advantages, provided a generally accepted set of expectations and rules. When the vows of fidelity were broken, it was clear who had cheated whom. In these days when such a high proportion of couples are unmarried, even if they have children together, leaving aside moral judgements, a major problem is that the relationship is undefined.
Exactly what each can and should expect from the other is often not discussed or agreed, and left very ambiguous. The Informal Relationship, like an informal settlement, has no by-laws and surveyor's pegs by which to assess its status.
There's another area of confusion in modern vaguer forms of relationships. As the informal relationship has no formal mechanism for dissolving it, the break-up is similarly vague.
Marriage assumes that your relationship with your spouse will be your primary relationship, that you are committed to keeping them as the most important person in your life. Most couples understand this to include the assumption that each will have sex only with their spouse. A majority of people in most societies believe that extra-marital sex is wrong, even if they do not always behave in accordance with this view. Studies in America in the 1980s found that about 70% of people felt that it was "always wrong", and only some 4% felt it was "always right".
The Kinsey Report
Yet the Kinsey Report (back in 1948) found that around 50% of all married males had some form of extra-marital sex at some time during their marriage; some 26% of females had had extramarital sex by the age of 40. Between the ages of 26 and 50, between 1 in 6 and 1 in 10 women were having extra-marital sex.
Not counting encounters with prostitutes, of men reporting on their first marriage, 71.6% reported no extramarital sex partners, 16.7% reported between 1 and 3, 5.6% reported 4 to 6, and 5.9% reported 7 or more. By comparison, of the married women, 79% reported none, 15.7% reported 1 to 3, 2.4% reported 4 to 6, and 2.7% reported 7 or more. Kinsey reported separately on sex with prostitutes, and 69% of white males reported having had sex at least once, with a prostitute. Among unmarried males, sex with prostitutes made up around 10% of their sexual experiences. Among married men, sex with prostitutes was never more than 1.7% of their sexual experiences (what Kinsey awkwardly called "their sexual outlet").
More recent statistics are much less reliable. In the 1970's a study of couples seeking couples therapy found the main problem was one or both having had an affair, in 46% of cases. A report from the 1990s found average rates of affairs of 35 - 37% for men, and 26 - 29% for women. Some have suggested the rate currently is nearer 50%.
Have I been unfaithful?
It is common within my online Forum for people to describe a range of behaviours, including loving but non-physical relationships with someone else, and to ask whether this constitutes infidelity towards their spouse or partner. I find the simplest test is this - if you feel bad about it, and feel you need to keep it secret from your partner, it is an affair. Even if the relationship has not so far found intimate physical expression, if it includes intimate emotional connection, it may well be an affair.
Though much of the focus of attention is on sexual aspects of affairs, research suggests that they are rarely only, or even mainly, about sex. They generally include strong emotions and feelings of love, and a desire to obtain from someone else a sense of being special, which the person no longer feels they get from their spouse. - (Prof M.A. Simpson, Health24's Cybershrink)