"in the heart - had he but known it - of Juliet's tomb"

This refers to Act 5, Scene 3 of Shakepeare's 'Romeo and Juliet', which takes place in the Capulet family's tomb.  Juliet has faked death and lies sleeping, ready to stir and be reunited with the exiled Romeo; unfortunately, he has not received word of this plan, and truly believes her to be dead.  He takes poison by her side, and dies.  Upon waking moments later, Juliet sees what has happened and stabs herself with Romeo's dagger, not willing to live without him.  Romeo and Juliet both needlessly die because of the enmity of their warring families.  In a similar sense, the girl in 'Lady of the House of Love' is unable to escape her fate, which has also been brought about by accident of kin.  The depiction of the scene below, from Baz Luhrmann's 1996 'Romeo + Juliet' has been modernised and edited, but the director's passion for opera lends itself to a grandiose moment of tragedy.