I couldn't find a definitive "correct" temperature at which to serve coffee. It seems that people typically want it at 60 °C when they drink it [2], but since not everyone wants to drink it straight away, often coffee shops serve it hotter than that.
[1] https://en.wikipedia.org/wiki/Liebeck_v._McDonald%27s_Restau...
[2] https://www.researchgate.net/publication/230106152_At_What_T...
The characterization of "melt the genitals shut" is not a precise medical, physical, scientifix statement. It's a colloquial, even poetic phrasing, and it does a FANTASTIC job of describing the impact of this woman's injuries.
The responder isn't telling the whole story though. The reason the coffee was so hot was because they wanted to limit the practical ability of customers to make use of endless refills on coffee. Not consideration for the average customer's wants.
In the future, maybe you'd be better served to trying asking the other person how they meant it, rather than jumping to correct them.
Unless your actual purpose is mainly to aggrandize yourself by dunking on someone else by correcting them... In which case you should just carry on, as you have been doing.