Here's how it works.
When a man and a woman are joined in flesh through sexual means, a flesh bond is created. As long as both parties remain faithful to each other, that flesh bond remains pure. Should it so happen that either party becomes unfaithful by having sexual relations with a third party, thus committing an act of adultery, that flesh bond becomes contaminated, or adulterated, from which the word 'adultery' is derived. Should it so happen that the faithful partner, whose half of the flesh bond incidentally still remains intact and untainted, decide to leave the unfaithful partner for someone else and joins himself or herself in flesh to that person by sexual means, no adultery is committed on his or her part as the person he or she was previously joined to in flesh by sexual means defiles it the moment he or she joins himself or herself to someone else in his or her moment of unfaithfulness. In other words, the faithful partner is no longer joined in flesh to whoever he or she was previously joined to the moment the unfaithful partner joins himself or herself in flesh to someone else.
Wanna know how many times you've committed adultery? Easy. Do the math.