Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. If the matter is as you mentioned here, that you enjoyed sex with your wife after the sunset then there is no harm since Allah made lawful for a fasting person eating, drinking, having sex, etc. from sunset till dawn. Allah Says (interpretation of meaning): {…
However, if one has sex with his wife or he enters the state of major impurity he has to take a ritual bath before performing prayers. No prayer is valid without taking a bath. Delaying a prayer from its fixed time is forbidden. Mere Wudu does not remove the major state of Janabah; one has to wash all his/her body with the intention of removing this impurity.
However, to eat or drink Suhoor (the meal a fasting person takes before Fajr) one does not need Wudu or Ghusl.
Allah knows best.