If you do find sex to be the most fulfilling part of your life, it might be worth asking where that sense of fulfillment comes from. It's probably a "who", not a "what".
Right. If its the only entertainment one ever has it could get boring. But its far more entertaining that... Most other forms of entertainment. That being said it shouldnt be the most fulfilling part of ones life.
There's a whole lot of real estate between "most fulfilling part of your life" and "boring".
If you find sex boring, you probably need to take a look at yourself or maybe find a different partner. Sex is rarely mind-blowingly great after your first few times but I can't say I've ever been bored by it.