I have to disagree about the 'scumbag' moniker, it's definitely a shitty thing to do to your partner in life, though I can somewhat understand. In a situation like that your partner is probably tired, not as sexually attractive, and with the right temptation I could see the frustration overcoming someone.
Before someone jumps all over my shit, I am not condoning anything he did, nor am I saying that it's excusable cause it's not. It is simply of my opinion that given his situation, plus the fact we don't know all of the gritty details (how has their relationship been over the last 3 or so years, was it for politics, ect), I can't call the man a 'scumbag'. Definite negative points on the respect scale for sure though.