Is she not wearing her ring?
Around a year after she left the house, Noah Fletcher made an open request to Don DeAngelo for the hand of his youngest daughter. Don DeAngelo ignored Fletcher.
Many speculated that Louis Fletcher was the mystery guy Bambina slept with as Fletcher tried to manipulatively force the don's hand into agreeing to his request.
Tony didn't know whether Marcella knew about the proposal. He couldn't face Marcella after "that" night. The don too was angry at him and avoided him. He didn't know what happened in the DeAngelo household at that time. He's only heard rumors.