You moan in pleasure and everything shifts. You feel lifted from your boring existence, at one with the cosmos. You can't even feel your body. You can hear the voice of your moaning, loud and passioned, growing higher pitched, and as the two of you pull away, panting, you blink and adjust your eyes as the world stops spinning. Your eyes widen and your heart skips a beat.
Standing in front of you, is.... you. You (or whoever is standing in front of you in your body) grins, but this grin, you can obviously tell, is not a friendly grin. Were you just... tricked?!
"Thank you for this body," your voice says, coming out of your body's mouth. "You may have that one. It was becoming tiresome. Besides, I could not leave this glade. Now that you're in my old body I guess that means you're trapped now instead. But that does not matter. You said you wanted my body, correct? Well, now you may have it as much as you wish. And I may do whatever I please with THIS one!"
You look down at yourself. You are clearly the dryad now, as your body is now that of the beautiful nymph that you just had sex with. And as the nymph walks out of the pond with your body in tow...