It's a double edged sword. Yes, sex can drain you personally of energy, leaving you feeling "high" afterwards but also drained, taking long periods of time to recover. It is also, as some of the others have said, a way to "share" your energy with another, blending so to speak, and in most cases, thier energy can replenish your now lost energy, though don't expect to use it the same way you would normally use your own. Just as Sundrop mentioned, you've actually intermingled your energy with thiers and it's a now uncontrolable mixture... giving you that virtual "high" afterwards.
One thing you must be wary of is being drained. Some people are natural givers while others are takers... meaning that if you are with someone who is a taker (leech), you may find yourself completely drained of your energy, leaving you exhausted and even confused. This isn't really a malicious act, but more of wrong place, wrong time, wrong person type scenario. And if you happen to the taker, please be careful of your partner's energy. Hope that helps.