assuming they legislate and represent their constituents interests in the government, why does it matter what legal things they do in their private life?Because we elect them to positions of power over our lives to legislate and allocate our nation's resources and otherwise represent our interests. If we don't demand ethical and moral behavior by those representatives, we have no guarantee they won't betray our interests, and our democracy, our freedom and our very Constution are all false illusions.
I don't think texting some girls from craigslist is an indication of a willingness to betray democracy.