Instead of the tickets, we see the tickets provider offering a set of keys of the lobby in the Theatre or Opera House. Theatre 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 gallery every night in its
Opera and Comedy theaters, at different prices"