I couldn't see what was wrong with the slogan at all, and kept reading the thread until some posters started explaining and giving examples of the other jokes, and someone linked to an explanation of what "attitude" meant in ballet terms.
I could see the innuendo straight away in the other phrases like "do it with balls", "do it in the mud", "do it standing up", "do it with fingers", "do it across the table", etc, because those are words that I can easily see associated with sex and sexual positions. Whereas "attitude" didn't mean anything else in particular to me in relation to sex, other than the normal meaning of attitude.
I thought this would be because English is my second language, and I wasn't living in an English speaking country during the 80s. But, interestingly, I showed the picture of the pink top to DH (who is a native English speaker), and he had no idea either what was wrong with the phrase "Dancers do it with attitude".
I think that now that I have learned about the double meaning or innuendo of the phrase, I wouldn't buy that top for a child. But I could have easily bought it by mistake before I read this thread.