I think even in marnameow's examples there's a problem with the listener - compare what al_fruitbat says above about being irritated by the use of non-gender-specific language. (Not meaning to imply that Al's attitudes are necessarily a 'problem', but that he and Marna would have a problem to overcome if they were to meet ;-)

Your 'tart' example is interesting, though, because as you say, it's not that the word doesn't exist, just that it carries different shades of meaning depending on the gender of the person it's applied to: there's no value-neutral word for a promiscuous woman, and no negative-value one for a promiscuous man. (Of course, I've just surmounted that problem by using two words in each case, but part of the reason that works is that I can be reasonably certain that you share my opinion of the relative merits of male and female promiscuity ;-)