You might have heard this already, but Jesus said if a man looks upon a woman in lust, they have committed adultery already in their heart with her. So yes, it is a form of cheating. Granted, it is not as serious as say actually sleeping with that person, but it is still wrong, it is still a sin, and it is still considered adultery (and the type of sin that will cause someone not to inherit the Kingdom of God). For we have to guard our hearts and souls and keep ourselves pure if we want to truly follow Jesus.
How can we say it is cheating? Because is not giving that sacred moment that belongs to one's spouse to them alone. Sex is only supposed to be within the bounds of marriage or for your life partner. Venturing outside of that is a form of cheating (i.e. adultery).