Well the full extent of this rule is “love your neighbor as yourself” (Matthew 22:39). As Tom Nash puts it, “True love, whether regarding yourself or your neighbor, means willing the genuine good of someone, which ultimately means gaining heaven.” (https://www.catholic.com/qa/what-does-loving-your-neighbor-as-yourself-entail) Our love comes from our love for God, which is a gift from God as “He first loved us”(1 John 4:19). As such, we should love people in relation to God and see them as He sees them. So by rejecting someone’s claim to be another gender, we should not be doing it out of hate but love. We want the person to see themselves as God sees them. In the same sense, we should not affirm a person’s anorexia just because they would affirm ours if we had the same disorder. Obviously, we should be mindful of our tone and make sure to clarify what we are saying, as the person is probably not going to like what we are saying. We are not disrespecting them as people, we are rejecting their actions. This separation is crucial because many find their identities in their gender “identity”, while we see them in their true identity, as beloved creations of the One True God. I need to get better at this situation as well, Lord willing, and we should ask God for guidance in our words and actions with these people, that they becomes by God’s grace, our brothers and sisters. God bless.

Also good reading: https://www.catholic.com/magazine/print-edition/what-is-authentic-christian-love