Very difficult question.
But, in a court of law, I could not be found guilty until I actually lay my hands on her delicious flesh with a sexual intent.
Ohhhhhhh that first grab of that first cheat, smothered with guilt, raging with emotion, the culmination of possibly years of endless fantasy, torture, and unfulfilled desire, all vindicated by finally sinking my teeth into that forbidden fruit.
But, outside the court of law, the cheating started years prior. Not exactly with the first thought of fornicating with your SO’s younger sister, older sister, her mother, her best friend, and eventually any female who is roadworthy.
For you cannot be responsible for random thoughts and desires that spring up from a bottomless pit. The actual responsibility for cheating kicks in when you grab one of those sexual thoughts which initially only exists in a seed form, full of potential, and start to fantasize, brood over, and feed it with your thoughts, emotions and desires. And, then interacting with your desires and fantasies, until it becomes a powerful whirlwind of raging desire bound for freedom of expression and adulterous activities.
Once you create that relationship with your sexual thinking process, and visit it often in thought, and make it your constant companion; since thought is father to the deed, it is only a matter of time before you bust out of your cocoon of faithfulness, and enter into the vast sea of sexual odyssey, and start to commingle with all manner of foreign flesh.
But, legally, once you squeeze the fruit, you then are officially labeled a cheater, and not in the thinking, planning and development phase.