You do not have to give them the domain. Youd definition of PlayBoy is correct and it may not have anything to do with the PlayBoy TM. It will cost you in defence fees though, so it is better just to let it go.
One thing you can try is develop it quickly and make it a strictly talk forums site devoted to 'players' and 'playboys'. Create your own logo, do not use anything wich would resemble the bunny and put a disclamer on the bottom line stating you have nothing to do with Playboy the enterprise and are in no way associated with them or anyone associated with them.