How does Lord Capulet respond when he knows Romeo is at the party in Shakespeare's Romeo and Juliet?

When Lord Capulet sees Romeo at the feast, he wants nothing to spoil the evening's gaiety, so he asks the heated Tybalt to calm down and ignore Romeo.

Noticing Tybalt's ire, Lord Capulet asks his nephew, "Why, how now kinsman! Wherefore storm you so?" (1.5.59). He tells Tybalt to calm down, but Tybalt contends that Romeo has come to make a mockery of their celebration.
Oddly enough, Lord Capulet here acts as the voice of reason, whereas earlier in a public place, he himself has engaged in conflict and sword play. However, Lord Capulet has heard of Romeo as being of a noble and sanguine personality; therefore, he wants no harm to come to him, nor does he want any conflict to disturb the happiness of the night. So, he instructs Tybalt to leave Romeo alone. But, Tybalt counters that being agitated when one such as Romeo enters is the right way to behave,

It fits when such a villain is a guest.

I’ll not endure him. (1.5.74-75)

Tybalt's words enrage Lord Capulet, who demands that Tybalt tolerate Romeo; however, Tybalt does not want to be tolerant. Trembling from the rage that is being suppressed within him--although he obeys Lord Capulet--Tybalt decides that he will later make Romeo pay for this intrusion:

I will withdraw. But this intrusion shall

Now seeming sweet, convert to bitterest gall. (1.5.91)

It is fateful that Lord Capulet wishes to not harm Romeo because by not doing so, he has allowed a situation in which Romeo can meet Juliet, and has also angered Tybalt so much that he vows revenge against Romeo.