I think we all agree that he opted in to vouching for the people. The complaint is that it wasn't clear that vouching also entailed sending a message if you didn't opt out of the message portion.
> FounderDating asked me to identify ten that I would vouch for. I didn’t have to do it but I thought oh well, I guess I’ll do it and see what happens. So all I did was click their photos. Nothing more.
Can't speak to the rest of the flow but clicking photos indeed sounds like opting in.
Yes, he opted in to "vouch for" those people. A ethical UX designer would have shown a modal showing the full message with the list of people it would go to BEFORE sending it on the user's behalf.
He clearly did not give his full consent because the UI was designed to bury the thing that he would have explicitly opted-out of.