Lady Capulet speaks the line "I would the fool were married to her grave" [3.5.143] after she has an argument with Juliet because Juliet refuses to marry Paris.
LADY CAPULET: Marry, my child, early next Thursday morn
The gallant, young, and noble gentleman,
The County Paris, at Saint Peter's Church,
Shall happily make thee there a joyful bride.
JULIET: Now by Saint Peter's Church, and Peter too,
He shall not make me there a joyful bride!
I wonder at this haste, that I must wed
Ere he that should be husband comes to woo.
I pray you tell my lord and father, madam,
I will not marry yet; and when I do, I swear
It shall be Romeo, whom you know I hate,
Rather than Paris. [3.5.115-126]
The irony is that Juliet is already married to Romeo, and she's saying that she hates him to mislead Lady Capulet as to that fact.
Lord Capulet enters the scene, and he wants to know if Lady Capulet has told Juliet what he considers is the good news about marrying Paris.
CAPULET: ...How now, wife?
Have you delivered to her our decree?
LADY CAPULET: Ay, sir; but she will none, she gives you thanks.
I would the fool were married to her grave! [3.5.140-143]
Lord Capulet is infuriated at Juliet's rebellious attitude. Juliet begs on her knees to be allowed to explain her feelings to her him, but he has no interest in what she has to say.
CAPULET: Hang thee, young baggage! disobedient wretch! [3.5.164]
Lady Capulet says "I would the fool were married to her grave!" in anger, but it's a dreadful and abhorrent thing to wish her only child dead simply because she doesn't want to marry Paris.
Clearly, Juliet's relationship with her parents is strained. The Capulets are not particularly loving parents. They don't understand Juliet, and they have very little patience with her. They want her to marry Paris for the status and wealth it will being to their family, and they have no concern about Juliet's feelings in the matter.
One interesting note about Lady Capulet's line is that it foreshadows what ultimately happens to Juliet, who is "married to her grave" all too soon.