Is infidelity a matter of thought or action?

What do you think is the point when someone is being unfaithful: when they decide to cheat or when they actually do?

Would you be more hurt by someone planning out and consciously making a decision to cheat but not going through with it (perhaps the plan fell through, they almost got caught, etc.) or by someone who actually went through with the action but with no forethought (a drunken one night stand or something of that nature)?

Would love to hear everyone’s thoughts on this. Cheers!