How does Lady Capulet show disrespect towards her husband in Act I of Romeo and Juliet?
Following the opening fight in scene i, Lord Capulet calls for his long sword so that he can join the fight. Lady Capulet responds "A crutch, a crutch! why call you for a sword?" Her statement is certainly dismissive of Lord Capulet's desire and ability to fight at his age.
We see in this exchange several important elements of these two characters. We see Lord Capulet's short-fuse and temper. These, of course, become important in Act III when Lord Capulet berates Juliet. Lady Capulet is also shown here to be rather sarcastic and not always submissive toward her husband's will and desires. This, too, is an important character trait to note and come back to in Act III.
I do not know that I would call it disrespect, but I guess you could say that it is... Anyway, at the start of the play -- in her very first lines, you can say that she is being disrespectful.
She and Lord Capulet have come upon the fight out in the street. He calls for his sword. She tells him he had just as well call for a crutch as a sword. She is telling him he is too old to fight.
To me, that's pretty funny and it is the sort of thing that a wife should be able to say to her husband, but I guess you can say it's disrespectful.