dont think its illegal but your conversion rates wont be high at ll... if you site doesnt have adult content then they will leave.. thats a bad idea i feel
I also think its a bad idea. Google is getting stricter day-by-day, and I donno, a day will come when google starts closing adsense accounts that get backlink traffic from these "no-no" websites....
i think its allowed, you can read adsense TOS although i've heard some stories google banned adsense account because traffic from warez/adult sites..google is weird..we never know..
Usually they have no much time to see them but If they find it then maybe you have to pay For that (may be get banned Sorry!!).