When discussing this topic things can get dicey, so for the record:

• Intersex people are not inherently non-binary
• Intersex people are not a mythical third sex
• Intersex people can be male or female. This is not an argument against allowing intersex people to identify with a binary sex or gender.
• When I use the term "sex-variant," I am talking about any situation where someone's sex characteristics are outside of the norm. This includes a lot more people than one might expect. It is not synonymous with intersex, as it can be used for people with different sex characteristics from artificial or environmental changes.
• When I say sex variant, I am including anyone who falls outside of the typical expectations for male/female. This can be intersex people, trans people who have received gender affirming care, cis endosex people who have had injuries or medical treatment that have lead them with different sex characteristics (getting a hysterectomy for instance), and anyone who can fall inside the intersex category even if there's debate whether or not they can claim intersex (PCOS, infertility, gynecomastia, hirsutism, etc.)
• Trans people who do not take any steps towards transition are transgender. The only reason why they will recieve a lot less focus is because I am focused on percievable sex characteristics. They are always valid, and I hope I can end up including them more as I learn more.

A lot of the time the idea is that if there is no third sex, then sex cannot be non-binary. This is a simple misunderstanding of what a binary is.