Just read a bit more about adult only games on wikipedia and the list is very short because Microsoft, Sony and Nintendo don't allow AO+ on their consoles. GTA San Adreas had originaly a mature rating but after the game was released people found a script that would unlock the hot coffee mod. It was named the hot coffee mod because your girlfriend would invite you for some hot coffee after you brought her home. Then you will have sex with her with your clothes still on. It was a scandel in the US because the mod was discovered after the ESRB rating. After a rerating it was AO+.
Also looked up the avarage age people loose their virginity. In the US boys loose their virginity on average at the age of 17.0. Non religious people lose their virginity on average at the age of 15.9. Apparantly people in the US have on average much sooner sex then they are allowed to watch these animated pictures with clothes on in the hot coffee mod.
Because of the policy of MS, Sony and Nintendo regarding AO+ games, it is safe to say Watch Dogs will be rated Mature (17+)