This has been a long running problem, for example, in UN bodies that work in developing countries and in the peacekeeping forces. UN peacekeepers in places like Bosnia in particular were accused of such crimes. It is appalling and the last thing that people need from third world charities is further abuse of desperate people.
That said, this story in the Times digs up an old case that Oxfam dealt with by dismissing the staff. Oxfam has been accused of being political by the Right for highlighting global inequality and this is presumably payback - the Times is owned by Murdoch, one of the global plutocrats accused by Oxfam.