It would be "his business" IF he were a private person. But he chooses to be a public person, chooses to be a representative of the people, and that's what makes his private business OUR business.
The fact is, someone who screws around outside of marriage, or cheats on their loved ones by having sex with others are not thinking of others as much as they are thinking of themselves and their own self interest and greed. We don't need to be electing more greedy people as our leaders, God knows how much trouble that has already caused PNG. Someone who looks after their own self interest by womanising is just as likely to look after their own self interest by stealing public monies and putting it into their personal pockets.
That's why we need to reject womanisers as our leaders. That's what happens in any country that is advancing today and is a free democratic society. They're moving ahead because they set very high standards for their leaders. Unlike us. Instead, we have lots of people like you who excuse these personal deficiencies and argue that we should keep our standards low.
You need to learn to think and analyse more deeply than a first grader if you want us to ever join the ranks of 'developed country'.