Hakatan, the “events, etc.” are not all mixed. Many are, yes, but it is absolutely and simply not a “co-ed experience”. You can spend many years at YU without ever being forced to socialize with the opposite gender, and those who believe that is improper to do so, don’t.
You should be aware that there isn’t a “klal” for YU’s hashkafa. There is a variety.
Last point: No one is claiming YU/MO is perfect or doesn’t make mistakes. There can be things done which are improper. However, to say that as a whole it is illegitimate (or Amalek, or AZ, or a deviation, or whatever else) is what I take issue with.