Sexual dreams are almost always not about the actual physical act, but rather the exchange of energy. If it's a coupling, even with someone you wouldn't have sex with, it can indicate a exchange of energy or merging of energies or simply absorbing the qualities you see and admire in another. If you're the only one performing a sex act, it could indicate that you feel you're servicing another or at their service. The same if someone is doing so for you.