Calling someone a terf is not insulting them, it's calling them out for their beliefs. One of the biggest signs someone is a terf, is when they take offence to the term TERF.
People who take offence to the terms like terf or racist, fascist, misogynist etc usually are yes. I'm not saying that if someone calls you a terf and you disagree you are one. But if you have issues with the term itself, then yes it's definitely a red flag.
Calling someone a terf is not insulting them, it's calling them out for their beliefs. One of the biggest signs someone is a terf, is when they take offence to the term TERF.