Answer: The easy answer is the Thinking – Feeling difference because your logical decision-making approach is what others hear you using when you speak. However, your Feeling values for your type are introverted (Fi) so they are lived rather than expressed outwardly. You treat people as you want to be treated, hopefully with respect and fairness, but it often is not discernable to others.
Your son-in-law has a preference for Extraverted Feeling (Fe) which means he values building relationships and behaving according to the norms of society to get along with people. This requires tack and diplomacy that your behavior may not exhibit.
I would not recommend trying to defend yourself because Fe and Fi represent true differences in perspective of what is important and the consequent behavior. A simple statement of what you value may be enough to settle that argument.