I have sex in the morning to wake up. Having sex doesn't mean you are supposed to role over afterwards and fall asleep. Sometimes I sleep after sex at night, sometimes I feel wide awake like you do. Don't worry about. Maybe you should continue having sex until you can't go on any more. Maybe then you will sleep.
Avoid stimulants before bed, if sleep is what you want to do in bed. Also clear your mind of whatever goes on during your daytime life, unless it's sex
That cuddling that Brandye mentioned, will help put you to sleep too.