“If a fasting person becomes junub as a result of a wet dream or intercourse, he does not have to do anything, so long as that intercourse took place before the adhan of Fajr. This is indicated by the fact that Fajr would come in Ramadan when the Prophet (peace and blessings be upon him) was junub not as the result of a wet dream, and he would do ghusl and fast. Narrated by al-Bukhaari, 1926; Muslim, 1109, from the hadeeth of ‘Aa’ishah (may Allah be pleased with her).”
Based on this, if husband and wife make sexual intercourse before the Fajr prayer, then their fast on that day will be valid. However, it is advised to always hasten to perform ghusl because everyone should pray Fajr on time.