Well, there are variations on how this situation is handled, depending on the local elders. I mean, my wife and I got hauled into a committee months after we were already married, for premarital sex. And I take it we were to prove our repentance by what, not having sex anymore as husband and wife for awhile? Well, don't worry, we covered that part already...ha ha...
But in her case, she got off with private reproof and was commenting at meetings like 2 months later. Of course, that was also probably an extra reward for turning me in for having doubts about the organization, but there's no way to know for sure--I think they assumed I was the instigator and seduced her or something, when it was actually quite the opposite...
That said, it does seem strange that they would make that decision based on not having a marriage date planned. Isn't the whole point of repentance that you stop committing the sin? I mean, did it only happen once, or several times? If it was several times, maybe they're just doing a sort of preemptive strike because they're expecting to keep doing it before marriage whether you get married in the future or not. Either way, well, that's elders for you.
--sd-7