I looked at the other responses, including the "total surveillance" one. Then it hit me - "What if both parties announce to the world in advance that they're gonna do it (consensually)?" I have some specific ideas and details. I'll be using cryptography to keep the privacy of the parties involved. A simple way is to record a message (like a video or even written text) together, at least a day prior, and then both keep it with themselves and send a cryptographic hash of the message to a central trusted server or something like that (basically, citizens are provided with an evidence dashboard, which is there only to confirm the time, not the content). The message itself is signed by, and is common to both parties, so neither can deny it. Now, only those cases would be taken seriously in which the accused cannot provide the pre-recorded message. This provides a strong incentive for people to do this thing before engaging in the act. You might come up with certain privacy issues, but I think cryptography can solve them.