My first instinct is to go with the fact that it is none of the public’s business and I do happen to believe that some journalists are like parasites and will do anything and everything to get a story and intrude on people’s privacy and whatever else they can get into.
Now, looking at this from another perspective, if you’re a politician, you’ve placed yourself in a profession where you are probably going to end up in the limelight at some time or another and eventually you will be susceptible to public scrutiny. These factors should indicate to the individual that they should live up to a standard that is moral and ethical (yes, I know what you’re thinking and you are right, there is no morals or ethics in politics) and you should conduct your lifestyle where you won’t bring discredit upon yourself. Knowing this, if the person eventually ends up in the news with a tarnished reputation because of their actions, at least half of the fault lies with the politician that screwed up in the first place.
Lastly, if someone is having an extamarital affair, politician or not, that is poor judgement all the way around, in my opinion. If things are that bad in a relationship, you should take whatever steps are necessary to repair the relationship or seperate from your partner amicably instead of having an affair.