Jim McGrath brings up a point I hadn't thought of. In Genesis, Adam and Eve are literally 'one flesh'. They're one individual who's been cut in two to make a sort of prototypical human community. In Mark 10, Jesus is made to say that a man will leave his parents, be united to the wife, and they will become 'one flesh'. The passage has to be metaphorical, since Adam and Eve start as one, and become two, while any other couple start as two and (hopefully!) become one. So the use of the story in Mark is rather more sophisticated than you normally get from the sort of fundamentalist preacher who I remember banging on about 'leaving and cleaving'.