Simple and plain. He will most-likely cave in and tell the Elders EVERYTHING. I've seen it dozens of times, although it is rare for a "man" to have this much guilt on his shoulders, but...oh well. Because you are a sister and you are both engaged to one another, your punishment...because there will be a sentencing (LOL) ...will only be "private reproof". You may be restricted from commenting at the meetings until after your wedding, and he will most-likely not be able to comment or carry microphones, man the soundsystem etc. for as long.
NOW - since you're having doubts about the ORG...I must ask you to consider all of this very seriously. You could get married and regret it, eventually being labled an apostate and being DF'd somewhere down the line with him resenting you wishing he had never done those "sexy things" with you and Jehovah, or Satan, is now paying him back for that night with...well...YOU. OOOOOR, you can call off the wedding, which is less painful than divorce, fade from the society, and live a happy life free from guilt.