Going to a web site is already opt-in. Plus google has default safe search. The only other path I can think of offhand is advertisements on torrent sites. If you want to make those opt-in I won't complain.
And as surely as you could "accidentally" visit a porn website (does this really happen to anybody? I am convinced that this is a myth created by the go-to excuse of every teenager and husband with a controlling spouse...), you could accidentally visit a restaurant that serves alcohol.
If such an incident disturbs you to the core, then turn around and find the door.
If such an incident disturbs you to the core, then turn around and find the door.