This may come as a shock to some people, like when a teenage girl realizes that the supposedly nice guy who told her that he really really loves her and wants to be her friend really doesn’t love and care about her but just wants something else…
If there is no mechitza and other gedarim, this is what will happen: the guys will check out the girls and flirt with them. If she is seen to be ‘under the influence’ and partying, she will be seen as an ‘opportunity.’ I know because I’ve seen what it is like in coed MO schools.
Do not think that just because a guy has a hat and jacket (or even big beard!) that he is any different than any other guy inside. Yosef was a great tzadik and this was still a big nisayon for him. It may seem horrible and not very nice but this is the way things are so that a couple, that does things the right way, will have a happier marriage together.