2012, Sergius Bulgakov, Unfading Light: Contemplations and Speculations, Wm. B. Eerdmans Publishing (→ISBN), page 308:
Among the innate and the voluntary eunuchs belong people “of the third sex,” both men and women, who recognize amorousness or “spiritual matrimony” but who disdain marriage and especially the generation of children […]
2012, Wayne Gray, Homosexuality, the Bible, the Truth: The Bible Does Not Condemn Homosexuality (→ISBN), page 117:
For a man to act as a woman was considered disgusting, but for a man to treat a eunuch, another male, as a female was not, since the eunuch was considered a “third sex.”
2015, Paul Chrystal, In Bed with the Romans, Amberley Publishing Limited (→ISBN)
He used to say that eunuchs were the third sex of the human race, one not to be seen or employed by men and scarcely even by women of noble birth.
2018, Christopher Fletcher, Sean Brady, Rachel E. Moss, Lucy Riall, The Palgrave Handbook of Masculinity and Political Culture in Europe, Springer (→ISBN), page 65:
He then specified that a eunuch at birth was an 'ambiguous' (amphibolon) creature. The eunuch was thus perceived as exterior to the two fundamental sexual categories, which made it possible to define eunuchs as a third sex, ...
