That Friar Laurence keeps the marriage a secret is assumed, even when he is confronted with the debacle following the violence in the streets which claims the lives of Mercutio and Tybalt and results in the banishment of Romeo. The Friar's solution to Romeo and Juliet's separation is to keep the marriage confidential until some time has passed and tempers have calmed. He says, in act 3, scene 3:

Go, get thee to thy love, as was decreed.
Ascend her chamber. Hence and comfort her.
But look thou stay not till the watch be set,
For then thou canst not pass to Mantua,
Where thou shalt live till we can find a time
To blaze your marriage, reconcile your friends,
Beg pardon of the Prince, and call thee back
With twenty hundred thousand times more joy
Than thou went’st forth in lamentation.

Friar Laurence also hides the marriage from Count Paris when he visits the Friar to seek assistance with Paris's own marriage to Juliet. Laurence is desperate to keep the proceedings from going forth. He tells Paris, "Uneven is the course. I like it not." Moreover, in an aside at the outset of act 4, unheard by Paris, Laurence suggests "I would I knew not why it should be slowed," meaning he wants no business marrying Juliet twice. Later in act 4, he again hides more than just the marriage when he comes to the Capulet house after Juliet has supposedly turned up dead. He acts as though Juliet is in heaven and that she is better off:

Heaven and yourself
Had part in this fair maid. Now heaven hath all,
And all the better is it for the maid.
Your part in her you could not keep from death,
But heaven keeps his part in eternal life.

Finally, after the Friar's plan has collapses, he is summoned to the tomb where he tells the entire story of the young couple's clandestine marriage, his attempts to reunite them, and his failed attempt to get a note to Romeo informing him of Juliet's faked death. He tells the Prince:

Romeo, there dead, was husband to that Juliet,
And she, there dead, that Romeo’s faithful wife.
I married them, and their stol’n marriage day . . . Then gave I her (so tutored by my art)
A sleeping potion, which so took effect
As I intended, for it wrought on her
The form of death. Meantime I writ to Romeo
That he should hither come as this dire night
To help to take her from her borrowed grave,
Being the time the potion’s force should cease.
But he which bore my letter, Friar John,
Was stayed by accident, and yesternight
Returned my letter back.