Parent's point was that it wouldn't be acceptable when done regarding some protected group (ex. blacks vs whites), so it shouldn't be acceptable when it comes to men vs women
Especially when the world never stops turning and social change / idea propagation move orders of magnitude faster than they did for most of human history.
Consider one of the (largely liberal) discourses rising in the past few years that many vital social metrics and trends indicate men/boys are screwed and women/girls are outcompeting them and thriving much better in present era.
There is no reason to think that interpretation of metric that compared "men" and "women" must be aligned with that same metric when men/women is substituted with some other populations. When you make the substitution you completly change the nature of the comparison. Whether either comparison is "acceptable" depends on the metric and the populations. There is no ethical rule that says all comparisons for group X must be acceptable or all comparisons for group Y must be unacceptable.
Then that's a lazy point, because it should be immediately obvious that studies for gender can be conducted easily across time and space whereas most demographic splits being discussed in this thread cannot because they are highly dependent on time/space and have therefore have endless variables that cannot be controlled for.
