To accept it, you just move on with your life. Find the next thing you should do and do it. The more you dwell, the harder it will be to stop dwelling, so just break the cycle and go do anything else.
You will encounter plenty more people who are insistently wrong. Each one will affect you less than the last.
Btw, there’s a deep annual survey at https://www.gendercensus.com . It’s not specifically about this community, and excludes purely cis-binary people, but has interesting data & trends if you’re into this sort of thing.