You can’t always know what someone’s pronouns are by looking at them. Asking and correctly using someone’s pronouns is one of the most basic ways to show your respect for their gender identity.
When someone is referred to with the wrong pronoun, it can make them feel disrespected, invalidated, dismissed, alienated, or dysphoric (often all of the above). All major professional American psychological and psychiatric associations recognize that inclusive language usage for LGBTQ+ youth and adults drastically decreases experiences of depression, social anxiety, suicidal ideation, and other negative mental health factors.
It is a privilege to not have to worry about which pronoun someone is going to use for you based on how they perceive your gender. If you have this privilege, yet fail to respect someone else’s gender identity, it is not only disrespectful and hurtful, but also oppressive.