Sesto has a lot of traits that are traditionally associated with female gender: he's sacrificial, loving, sensitive, tender and very emotional. And there's something more, something I can't quite put my finger on but definitely feminine. It echoes very vividly in Sesto's music, especially in the above-mentioned Deh, per questo istante solo. That's why I think that this role must be played by a woman, and mezzo tessitura has nothing to do with it. No ordinary man would be able to convey this character convincingly and sympathetically.
Interesting: "Glad to be gay" (the article in 'The Guardian' about operatic gender-bender).