The first option of expiation for having full sexual intercourse between spouses during obligatory fasting hours is to free a slave (when there was slavery). If the husband cannot do that due to any legitimate reason (like lacking slaves as it is the situation nowadays), then he has to fast sixty continuous days.

If the husband cannot do that either, he has to feed sixty poor people. As for the wife, she has to make up for that day according to an opinion attributed to Imam al-Shafi`i.

The above expiation is required in addition to making up for the day they missed.

http://www.onislam.net/english/ask-the-scholar/acts-of-worship/fasting/what-breaks-the-fast/487209-expiation-for-sex-during-fasting.html