There are a few options for recording intercourse data. You can use any one of them, as long as you are consistent.
You can go by a 24 hour clock and record any intercourse that occurs before midnight for the previous day and any later intercourse for the following day.
You can record any intercourse that occurs once you are in bed for the night for the following day. This method is the most natural since you don't have to go back to the previous day's data when entering information in the morning. It is often more accurate as well since the intercourse data entry is more relevant to the following morning's temperature than the previous one.
You can use the AM/PM options on the data entry page to indicate when intercourse took place.
Tap the Analysis tab to evaluate your intercourse timing, use the Intercourse Timing Analyzer.