[dragonwriter]

Your reading of "the problem with girls...when they are in the lab" requires the sexist assumption that the default state of labs is to include males, such that that a "problem with coed labs" is a "problem with girls...when they are in the lab", because, obviously, there's always going to be men in a lab.

So, either reading is sexist, though it is true that there are different ways in which it can be sexist.