President to what? The president of, say, the local art society only has artistic standards and values to extol so his marital status is really and generally of no consequence.
The president of a country has certain moral values to extol which brings in the question of marital morality, a president, if married, of a monomogomous country would, at least, have to follow that principle. Generally polygamous socities display an element of choice so it isn't essential to follow the code of polygamy and stioll be a role model for the country's values.
Of course the whole of that doesn't address the other problem - just because a country has a moral code of monogomamy, is that necessarily right? Another story methinks.