It usually begins with a lie. The first lie told is the one the cheater tells his or herself. It's the lie of "justification" which gives them permission to move forward. There are also other instances when the soon to be cheater tells themself their behavior or the circumstances they are involved in is "innocent" ( "He or she is just a friend", "We just flirt back and forth with one another" or "Since I know nothing is going to happen between us there was no reason to tell you about him or her", "It was just a kiss")
The bottom line is cheating involves lying directly, indirectly, or by omission. It also means keeping secrets or hiding behavior. The cheaters who live in denial during the early stages are often the ones who end up saying, "We did not plan this!" or "One thing led to another". Just as there is usually foreplay before sex there is also a type of foreplay before most affairs begin. Trivializing or justifying "secret behavior" one knows their mate would NOT approve of if often the first step towards cheating.
Having said that there are also people who never intended to be faithful to begin with.