What is the element of humor in the exchange of rings?

In act 3, scene 2, Portia gives Bassanio a ring to solidify their marriage and tells him that if he ever gives it away, then their love is doomed. Nerissa also gives Gratiano a ring, and he promises never to lose it. Later on in the play, both Portia and Nerissa disguise themselves as men and Portia ends up saving Antonio's life under the guise of a young lawyer named Balthazar. In act 4, scene 1, Portia, who is disguised as Balthazar, takes Bassanio's ring as payment for saving Antonio's life. Reluctantly, Bassanio unknowingly gives Portia the ring he swore never to give away, and Gratiano also gives his ring away to Nerissa at the beginning of act 4, scene 2.
In act 5, scene 1, Portia and Nerissa mess with Bassanio and Gratiano by questioning them about their rings. Shakespeare creates humor through the use of dramatic irony. Bassanio and Gratiano are unaware that Portia and Nerissa possess the rings as the two women play with their emotions. Bassanio and Gratiano are both heartbroken as Portia and Nerissa pretend to be upset and deny them access to their beds for giving the rings away. Portia then bets her life that Bassanio gave his ring to a woman, which he unknowingly did. Portia and Nerissa then reveal their participation in Antonio's trial, and their husbands are astonished to discover they actually gave their rings to their wives, who were disguised as men.