Sati gets the boon even though she could not ask for it

तदा वाक्यस्यावसानमनवेक्ष्य वृषध्वजः |
भव त्वं मम भार्येति प्राह तां भक्तवत्सलः || 15 ||

Seeing at the end of the sentence, Shiva who is kind to devotees said "become my wife".

When told to ask for a boon, Sati could not ask for it, being too shy. God knew what she wanted.