Religion, politics, professional sexism, all these are topics that infringe on people's identities, causing a strong gut reaction that leads to terrible discussions. Even programming languages can end up in a similar category, if you have too many people that identify as a "C++ programmer" or a "Pythonista", or a "Lisper", which leads to the oft-seen programming language flame war.
This is a good time to bring up one of my favourite Paul Graham essays, "Keep Your Identity Small".
http://www.paulgraham.com/identity.html
Religion, politics, professional sexism, all these are topics that infringe on people's identities, causing a strong gut reaction that leads to terrible discussions. Even programming languages can end up in a similar category, if you have too many people that identify as a "C++ programmer" or a "Pythonista", or a "Lisper", which leads to the oft-seen programming language flame war.