The old adage about never discussing sex, religion and politics is a platinum rule in conversational play. In tennis equivalency it is like throwing your racket, continuously arguing about a line call, and temper tantrums (unnecessary comments about shots/disrespect).
Why are these three subjects dangerous? Each subject contains strong emotional heritage - strong beliefs involving judgmental values. Even if you think you have flexible or open beliefs about any of these subjects, the other player may be totally the opposite. A conversation into any of these areas can easily fall into a trap of debate club and there is zero probability that you can convince someone in a short conversation to change their deeply rooted values and beliefs.
I might even add a fourth subject of danger - money, salary, and material possessions. Money, Sex, and Religion are the three major causes of divorce - another key indicator of conversational radioactive zones.
So even if you know someone as well as your spouse these subjects are volatile areas of risk and better left out of casual conversational settings.