My definitive mansplanation moment was the time a particularly ‘splanation-prone asshat argued with a female friend and I for a good hour. This (straight, cisgender) guy was well known for holding forth on all sorts of topics whether he knew a damned thing about what he was talking about or not. And he had to be right. All the time. I thought he was an ass before this conversation, but I could never take anything he said seriously again after he set this gold standard.
The mansplainer mansplained adamantly on the topic of sex. His specific sub-topic: whether a long penis or a wide one is more pleasurable for a woman during vaginal intercourse.
He knew best. What my friend and I, and the occasional gobsmacked woman passing by, thought about his opinions – well, that didn’t matter.
I hope I never meet a mansplainer to match him.