Why not let couples be alone unchaperoned? You can ask them whether they had sex afterwards and if they say no and there weren't two witnesses to say otherwise, then you can be sure it didn't happen - just like the JW beliefs with pedophiles. If they both admit they did and aren't sorry, you can DF them and keep your congregation clean.
Reniaa, the scriptures do speak against immorality. But it is your responsibility to train your mind to not let that happen. If you can't be put to the test, you're not really following the scriptures, are you?
The scriptures also speak against things like theft. Does somebody need to follow you around everyplace there's things you can steal? Is a chaperone constantly accompanying you to work to make sure you don't steal pens and PostIt notes?
This is as horrible as George W. Bush's pre-emptive strikes. In both cases, you're assuming a wrong WILL BE committed and taking action instead of waiting to see what will actually be done and punishing (if need be) later.
The JWs need to be unchaperoned even more than more liberal people. I've seen so many JW couples get married just because they wanted sex. They didn't get to know each other. They weren't even old enough to know who they were. Then they end up miserable in a marriage.
At the very least, JWs should get to really know the other person first (and I'm not talking sexually). They should know what they like, what they don't, how intelligent are they, how controlling is the other person, etc, etc, etc. A chaperone inhibits getting to know somebody. You don't want to get personal about yourself with some other dude sitting next to you hearing it all. Already in dating, people are on their best behavior. And they're not going to relax with somebody else there looking over their shoulder. They need some freedom.