So, Alice and Bob are making the two-backed beast, so to speak. Except it's more of a three-backed beast. With or without them being aware of it, a third party is actively enjoying their encounter, and not in the usual way, either. This trope is when: Please don't list examples of body snatchings that lead to sex (with the body's original owner left unaware), or examples of characters being involved in sex without realizing it (hey, it happens). Those are both different tropes. Examples of Sex by Proxy include: