"It's been so long since I've had sex I've forgotten who ties up whom"

In this quote, Joan Rivers is making a humorous comment about her absence of sexual activity. She is indicating that it has actually been such a long time given that she has had sex that she has actually forgotten the conventional roles and dynamics of a sexual encounter. The expression "who binds whom" suggests a dominant and submissive dynamic, with one partner being in control and the other being restrained. By stating she has forgotten who binds whom, Rivers is poking fun at her own absence of current sexual experience and playfully acknowledging that she may not remember the traditional roles and expectations in a sexual encounter.