Sex and politics have traditionally been the two topics that are best avoided in social settings. It seems that tradition is gradually disappearing.
It's like all those bar leagues where people play softball for 90mins and then hang out at the bar afterwards for the rest of the day.
Sex, Politics, Religion were the traditional ones to avoid. I'd also add 'childcare/how to raise children' as another one that can easily escalate/some people have very strong opinions on.