A website/app where you can quickly declare consent for a specific activity, and share that declaration privately with a specific other user of the app. Each entry is secure/private to only you and the person you share it with. The entries are timestamped, you cannot delete them.
Then before the act, you quickly do a consent exchange and each person has the proof that the other consented.
You could make the entries visible only for a period of time to their own users, after which they are only accessible if there is a legal process, to avoid them being used as trophies/outing.
Obligatory Dave Chapelle Love contracts: http://www.cc.com/video-clips/jwmvxd/chappelle-s-show-love-c...