Technically, the US Army (the senior service) realized this from the beginning (1775). It was only in the mid 1980s that politicians locked targets on the largest service and politicized General Officer careerists caved in.
Some day, we may rightly return to single-sex training for the sex-integrated branches.
I believe my platoon in Basic Training at Ft Dix in 1979 was one of the first male/female platoons.
They announced it to us. They also announced that we were to no longer call them "Drill Sergeant" but simply "Sergeant".
Oh, they were not happy about that at all. But it was the only slip-up we could do that didn't result in push-ups - to call them "Drill Sergeant".
Later on I just assumed this was more Carterization of the Armed Forces as things continued to go downhill from there.