For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
It gets much easier once you factor in that you, yourself, aren’t static and constant. The task isn’t to find someone capable of becoming perfect for you, it’s finding someone whose compatibility and willingness when taken into account with your own offers a fair chance to grow into a symbiotic relationship.