But again OP you are missing the point that a lot of people are making, for instance the way you word things is odd - the definition of a “fundamental truth” is:
The definition core, essential principle or fact that serves as a foundation for understanding, belief, or action, often considered universally and eternally valid, like "gravity exists,"
You are comparing YOUR feelings about YOUR experience in YOUR relationship with something that is a scientific, proven fact and applicable to everyone around the globe: ie: it’s a fundamental truth.
You are absolutely steadfast in your belief that you are right and everyone else is wrong or mistaken or lying. That the people who are finding what you are saying (ie assuming everyone else in a long term relationship is basically unhappy underneath it all, has the Ick and feels like they are shagging their brother) incompatible with how they themselves feel about their long term partners must be in denial.
That is what people are finding odd and taking exception to. No one is denying that you yourself feel that way, as that is your experience. But that doesn’t mean it’s everyone’s.
That is what is strange. Do you understand?