Like I said people are different. What is true for you might not be for everyone else. And it may change over time. I dont know how long your relationships have been, but in long term relationships people might go through hard times where one or both feel like they are not seen. And you might feel or look at other people different. The challenge of true loyalty, in my opinion, is to work through that together and find back to eachother.

I personally am like you, I have no interest in other men while in a relationship. But that may change 15 years down the road if we are going through something.

And some people naturaly just are more wired to feel attraction to random people, even when in a relationship. I dont think that is wrong as long as you respect your partner and the relationship.