The big idea is not that girls distract boys and boys distract girls, so they should be apart. The big idea is that boys and girls, especially at high school age learn/are motivated in fundamentally different ways so splitting them and teaching them in different ways could be more effective.
So this is not empirical evidence, but my mom who has been teaching high school for over 30 years strongly agrees with this. In her opinion the average boy just can't sit still and needs to be motivated more competitively, whilst girls benefit from more social engagement.
Of course that analysis is rather crude. There's subtleties there. Of course boys would benefit from being taught to be more social, and girls would benefit from being taught to be more competitive. But it's clear that there's efficiencies to be had here.
For me personally high school felt as a very inefficient time of learning, but I also had much fun and made many friends with both boys and girls. I wonder if the research has shown anything there. Do boys who graduate from boys-only schools work well with girls? What is their opinion on them? And vice versa?
Although I think high school could benefit from efficiency improvements, I also think high school is an important preparation for growing into a broad-minded adult. I hope they keep that in mind when the decisions are made.