I wouldn't say most, but a pretty large group does.
And, NO, these couples never come clean about it and go on to have weddings in the KH as if they were morally clean.
People aren't stupid, though. Most other JWs know when a couple has gotten too familiar with one another, ahem, if you know what I mean.
I told you all before about this young couple that got married last year in one of the local congregations. I knew it, just knew it that they're having pre-marital sex but of course couldn't prove it. Sure enough, 6 1/2 months later here comes a beautiful 10 lbs. baby boy. Of course the family all said the baby was born prematurely. YEAH RIGHT!!!!!!!!!!!!!!
The father elder and the mother pioneer both lost their privileges because it was determined that they knew of the pre-marital sex but covered it just to have the daughter have a lavish KH wedding with all the trimmings.
Time has passed and everybody is back reinstated to their privileges and everybody is one big happy family once again.
DY