THE nudity and violence that assail passers-by from giant posters outside some of the territory's most popular cinemas do no more than reflect - perhaps even overstate - the titillation on offer to those who chose to go inside. But it is the matter ofchoice which makes all the difference. Clearly those willing to part with more than $30 for the dubious privilege of watching the Category III movies advertised are unlikely to be offended by acres of bare flesh on the big screen. But it is hardly surprising that parents with young children, and the majority of adults who do not choose to take the bait, should be offended when Category III material is forced on their attention on their way to school, to church or to the supermarket. Such posters would be unacceptable in most ''decadent'' Western societies, despite objections to censorship of the arts. To find them displayed in a conservative community like Hongkong is extraordinary. The Television and Entertainment Licensing Authority (TELA) has taken a sensible and measured approach to the Association for Democracy and People's Livelihood's protest at this increasingly explicit visual assault. It has promised to consider asking, after public consultation, that cinema posters be submitted to it for approval along with the films they promote. In the meantime, cinema operators could practise some self-restraint. There are better and subtler ways of marketing their wares than by shocking the public with pictures that would have to be sold in a sealed wrapper if published in a magazine.