Ive been married for 19 years. I have never cheated on my wife and she has never cheated on me. Our sex is extremely intense and fulfilling, lustfull even.
The secret to not getting bored with the same woman for 20 years? I never mastrubate or fantazise to or about any other women than my wife. To the best of my ability I avoid looking at other women sexually or noticing them sexually. I discipline myself sexually. This leads to intense lust for my wife every single time we have sex.
I know this works because I have done it both ways with my wife, thought of other women, fantasized, porn ect. Every one of these acts diminishes the intensity I have with my wife. The lack of them intensifies it.