I recently heard a speech at a camp where the staff was told that in this place there is great kedusha and השראת השכינה. Very inspiring.
But the truth of the matter is that if you have unmarried boys and girls at the age where the hormones are raging, where the תאוות are at their height, and you have them interacting at all hours of the day and night for 2 months, it is anything but holy. Rashi [whose yahrtzeit is this week] says at the begining of parshas Kedoshim that where there is a separation between the sexes you find kedusha. Otherwise it is the opposite.
I am not minimizing the great mitzvos that people sometimes do in co-ed settings. I am in favor of mitzvos and especially of the mitzva of chesed. For that they have all of my respect and Hashem will surely reward them in this world and the next. But it is NOT Kadosh.
Kedusha is separation. Satmar does a ton of chesed in hospitals and the like but they do it without putting young men and women together on a constant basis. I would prefer to see a world where mitzvos are done bi-shleimus.
If someone tells me that there is a yeshiva where everybody learns at least 17 hours a day but it is co-ed then it is NOT holy. Learning is beautiful but so is learning bi-tahara. Chesed is holy but only chesed done with tahara.
We live in a generation where [as I noted in the shiur linked in the previous post] the boundaries of modesty have been breeched. Even in our religious world we hear again and again [and yet again] stories of people who have fallen in matters of kedusha. Nobody is immune.
The best way to ensure that we don't fall is to maintain a healthy distance between the genders.