When we respond to the transgender issue by pointing out that biology makes it clear that there are only two sexes, we are attacking a straw man.
Gender ideologues do not typically deny that sex is binary. What they deny is that our identity is limited to or determined by. In addition to our sex, we also have a gender and gender-identity. Gender is a social construct related to the different behavior and roles men and women play in society. Gender-identity is one’s perception and deeply felt experience of their gender. Most people’s gender and gender-identity matches their biological sex, but some people experience a mismatch. They may have a male gender-identity but a female body. In such cases, they should be considered a male because gender, not sex, gives us our identity. Some transgender people may desire to alter their body to match their gender, but others may be comfortable with their body. That’s why it’s perfectly legitimate to speak of men with uteruses and women with penises.