Answered by Shaykh Amjad Rasheed
If one wants to fast, and the dawn enters while engaging in intercourse with their spouse, is their fast valid?
If they withdraw as soon as dawn comes in, the fast is still valid. However, if they continue, such that they do not immediately disengage, their fast has been broken, as is in the Minhaj [of Imam Nawawi] with the Tuhfa [of Imam Ibn Hajar, the most important commentary on Nawawi’s work]. (3:412-413)
Amjad Rasheed