The Laboratorium : Sexual Spandrels

Here�s one for the biologists: is there an evolutionarily-preferred human sexual position?

So far as I know, homo sapiens is unique in having a choice of positions; dogs, for example, have only one, eponymous, option. Is this flexibility an evolutionary feature that plays a role in motivating us towards reproduction? Or is it a spandrel, an accident of anatomy evolved for other purposes? If so, is there a �natural� coupling configuration to which it is only our unruly brains have devised alternatives? Or is my premise simply incorrect, in which case which other species go at it in all sorts of ways?