FatFreddysCat makes a valid point about "agree" and disagree".
People refuse to accept that someone simply (disagrees) with them. Instead they feel the need to prove this person is "wrong" or somehow they are a moron for having their difference of opinion. Before you know it they have gone from having an intellectual debate to launching personal attacks.
When it comes to religion and politics people often become especially heated if you challenge their views or beliefs. No one wants anyone to "change their mind" or have someone "raise doubts" in their mind. Simply listening to an opposing view is unthinkable and to make any concessions that some of what another person says has some validity is a sign of "weakness" in your argument. Therefore it must be an "all or nothing" battle. "I'm right and you're wrong!". "I'm intelligent and you're stupid!"
There will always be those who like to "stir the pot" and their goal is to come up questions to try and push people's buttons on one side or another on an issue. Some people LOVE to fight, argue, and debate. The notion of simply agreeing to disagree is lost on them. I've seen instances where if you disagree with a person on (one thing) that is enough for them to ignore or disrespect your opinion on everything! They can only be friends with those are in lockstep with ALL of their views.