What happens when couples get intimate at night but don't do the ritual bath until very close to Fajr before performing the prayer? Would the fast have technically started by then and, if so, what is the consequence?
It is compulsory on every one of them to perform Ghusl of Janabah latest before Fajr (Dawn) time. Remaining deliberately on the state of Janabah will invalidate the fast and cause a major sin with all its sequences which are Qadha of the fast of that day and Kaffara of feeding sixty poor persons or fasting two months. Ghusl must be performed before the time of Fajr.
Wassalam.