If you believe they are involved in criminal activity and facilitate that in any way, whether you can wink, nod and look the other way may not protect you from at least being investigated.
Just how much do you mind being investigated by the state or the feds?
Creating a web site for an escort service would seem perfectly fine to me. Escort services are not illegal per se and there are legal ones. Is there something wrong with a man wanting a pretty woman on his arm for a dinner to impress his clients? Perhaps to be seen at the opening of a gala? Merely designing a web site is far removed from the picture as long as you're not knowingly a part of any extra cirricular activity.