I think you mean there’s only 2 sex, except there’s also intersex. Anyways go through the definition of they/them (you’ll find that they are pronouns for an unspecified gender) and also search what gender and sex is, and realize that they aren’t the same thing. Gender identity is something only humans have, no other animal has gender, just sex. Humans have multiple genders because of how they feel, such as dysphoria about male and female body parts. I for example do not feel comfortable in this body, so I wear a binder, however I’m kind of okay with my lower region because it is not noticeable. I however am also asexual because I would feel extremely uncomfortable because I do sometimes wish I had the other lower body part, because I’d be more comfortable having sex in that body. If you have any questions, want me to specify, or just want to share your opinion more in dept, feel free to message me.