I work in the sector (have never worked in Oxfam) and it makes me sad when people say they'll stop donating to charity because of things like this.
It's bloody awful, don't get me wrong, but as far as I can tell the situation seems to be Oxfam heard rumours of what had happened, had an investigation and got rid of the staff involved.
90% of people working in the charity sector are committed people, who could make more money elsewhere and are motivated and dedicated by what they do. 5% are a waste of space in one way or another, and some are attracted to it because it offers access to vulnerable people.
People who want to abuse women and children become police officers, priests, teachers, etc etc. They purposefully put themselves in positions of power over others. And all we can do is create a culture where when people around them realise, allegations are investigated and action is taken. I've only read one article about this, but it seems all involved lost their jobs, DFID were informed. I am guessing there wasn't enough evidence to press criminal charges, or it wasn't possible in Haiti.
I completely agree it's shocking. I worked overseas in a developing country and there were foreign ex-pats working for other aid agencies (not British) who were rumoured to use prostitutes. It's disgusting, and an abuse of power. It doesn't discount all the good work done by the organisation, especially if their own procedures caught this and stopped it. And it absolutely isn't the case that small local charities have better governance- I've worked for both, and it depends on the charity rather than the size, but larger ones are required to have a greater amount of oversight generally speaking.
The best thing people can do is ask the charities they support about their governance, read the policies, look at the external audits, see what guidelines they sign up to. External pressure on these points raises standards and is key.