I challenge you to find any trans man who cheerfully agrees that he is "biologically female," or vice versa. We don't exist for you to argue about biology. These statements are said in a way MEANT to undermine people's identities. While I disagree with your statement for other reasons as well, the main thing for you to know is that it's inappropriate to tell people what they are "biologically." It doesn't matter what you think about that. They say they are non-binary, they're non-binary. He says I'm a man, he's a man. She says I'm a woman, she's a woman. We don't need to get into the "biology" of things, just be respectful and kind. It's not about when we are discussing the construct of gender or the nature of biology, it's when people use that terminology to talk about a specific trans person.

For example, if someone you knew was a trans woman, and you said that she was a "biological male," you wouldn't be saying that to invite a curious conversation about gender vs. sex. People talk like that specifically to imply that "well, she says she's a woman...but she's biologically male. So we know the real story here." The more accurate term is "assigned male" or "assigned female." There are plenty of intersex people who have sex characteristics/organs that are associated with both male and female traits, and yet doctors typically "assign" them to be either male or female and will even alter their bodies as an infant to more closely align with the sex that the doctor sees as most appropriate. Additionally, no one uses the word "female" strictly to mean "someone born with a vagina." It is often used interchangeably with "girl" or "woman."