"The proportion of female mathematicians who are married to male mathematicians is much higher than the proportion of male mathematicians who are married to female mathematicians. So women must be going into mathematics to find a husband, while men do math because they're interested in it," male mathematicians have told me over the years, in all seriousness.
They agree.
I continue, "So the number of female mathematicians married to male mathematicians is exactly the same as the number of male mathematicians married to female mathematicians. The large difference in percentages is simply because mathematics is a male-dominated field."
They're astonished to realize that the numbers are exactly the same. There's something counterintuitive about it and it takes awhile to sink in, even for mathematicians.