Addition of a third party, either by mutual consent or secretly, into the intimate circle of a couple’s shared world can wreak devastation to a long-term relationship or marriage. Affairs are right up there with addictions and abusive anger as far as being one of the leading causes of divorce. Betrayal of trust and of sexual fidelity are made all the worse by secrecy, a point made well by Mark White in a post on this site on why adultery is so harmful.
That’s why I’ve posted thus far multiple blogposts on affair prevention. One article discusses resisting the three main temptations that destroy marriages. Another is on how the nature of sexuality contributes to inadvertent affairs. A third post is on keeping your marriage strong which helps to ward off infidelities from your marriage relationship like keeping your body healthy helps ward off infections.