Adultery disrespects the vows of marriage and destroys trust. Like any large lie, it must be covered by smaller ones and there is always the chance of slipping up.
Proctor cheated on his blameless wife, Elizabeth with the predatory young girl, Abigail. He feels very guilty, but won't cop to the indiscrestion...until he inadvertently is caught in the lie. Elizabeth and John both know Abigail is lying about the accusations of witchcraft and that innocent people are getting hurt. She knows that Abigail has told John personally that she is a fraud. Elizabeth begs John to tell the court what he knows.
John, however, makes excuses for not going, for he knows that to tell them would necessarily reveal his adultry. In trying to weasel out of going, he slips:
Proctor: I am only wondering how I may prove what she told me, Elizabeth. If the girl's a saint now, I think it is not easy to prove she's fraud, and the town gone so silly. She told me in a room alone--I have no proof for it.
ELizabterh: You were alone with her?...Why, then, it is not as you told me.
For this couple, adultery does not only destroy their lives, but contributes to the deaths of many others, and eventually John himself.