Gianciotto was always too busy.
Even to marry.
So Gianciotto married Francesca by proxy, his brother, Paolo, taking his place at the wedding.
Soon after, Gianciotto left for battle, and Paolo read together with Francesca.
Reading of Lancelot and Guinevere, their passion exploded.
One day, Gianciotto caught them making love.
And killed them.