Insurer denying coverage

My insurer has denied coverage of my operation. Their argument is the fibroid was pre-existing before I took out my policy 2 years ago. I only have my word to prove I didn't know. Out of curiosity, how common is this? I'd love to hear experiences of similar stories and if anyone knows of research into gender inequality in health insurance, please share. I am aware of healthcare disparities, so looking specifically for insurance TIA