Most Helpful Guy
Sex is an important component of most successful marriages. I wouldn't tie the knot without having figured this out with him first. He probably doesn't even understand it clearly himself, but if the spark is already gone before the wedding, it will definitely be a sexless relationship after marriage.
Most Helpful Girl
He's gaining something from this relationship, people only do things if it benefits them. and if it's not sexual, then either he will never be interested in sex and is basically asexual, or he's getting something else out of this relationship and he doesn't actually find you attractive..
Although to me it sounds more like an insecurity, he wants to wait until after marriage, or he just doesn't want sex. Just ask him about it, if you can't ask your fiance that question, who the hell can you ask?