What is this picture?
That would be yab-yum.  I think the wikipedia entry will more than answer your question.  You can find that here.  The one thing the entry only hints at, however, is that this is an actual practice in Tibetan Tantra.  In fact, it's generally what comes to mind, for a popular audience anyway, when the word "tantra" is used.  I think it's an exercise in futility to try and justify this practice according to Theravada or Mahayana ethics.  Within a tantric context, it just is what it is.