Instead of the tickets, we see the informal theater seats provider offering a set of keys of the boxes in the Theater or Opera House.
Theater and culture have always been a very strong tradition in Venice, and still are.
We don't know if the couple in the picture is married. They probably are, but maybe not with each other.
"In piazza de S.Marco semo avvezzi
fitar palchi ogni sera in sie teatri
D'Opera, e de Comedia a varij prezzi."
"In San Marco Square we are used
to rent a box every night in six theaters,
Opera and Comedy, at different prices."