Doctors won’t change my due date!?

At my 8 week ultrasound they said my baby was only 6 weeks. I instantly knew this was wrong because we had sex ONCE that cycle and that was the day before ovulation. I could understand if it was off by like 4 days but that’s it. There’s no other way. A week later we had another ultrasound due to bleeding and baby had a growth spurt they said and was measuring closer to my dates. They are still going with my first scan dates however. At my 20 week scan they said I have a BIG baby. He’s measuring EXACTLY on my LMP dates. But according to them he is measuring 2 weeks large according to their dates. Now I’m 26 weeks by my LMP and they are doing ultrasounds every 4 weeks to make sure baby’s growth is okay and they’re talking about doing a c section because they’re convinced that my baby is going to be massive. Quite frankly I will not get a c section because I know they are wrong about my dates. I had a natural birth the first time and I strongly intend on getting another one unless it’s completely necessary and I do not find it necessary. It’s frustrating that they’re not listening to me. I do not want a c section. I had very strong plans for this labor and unless baby is in danger I do not want a c section.