Sex within a marriage is usually pushed down to the bottom of the pile importance to have a successful happy marriage
Aside from maybe 3-4 references to the scripture that says that wives and husbands must render each other their due, I don't think I've ever heard sex listed as something that was a part of a happy marriage in my entire life in the cult.
Essentially everything said about sex in the cult is negative - STDs, unwanted pregnancy, ruined relationships/friendships, your marriage won't last if you have sex first, even if you do get married after you have sex you'll always regret not saving yourself, and on and on and on. In such an environment, is it surprising that there's rarely a healthy view of sex to be found?
Another way that they lead the youth into the trap of early marriage is by restricting dating to only those who are actively seeking marriage. For one thing, this immediately puts marriage in the discussion at the very beginning of a relationship - it's always in the mind and it seems inevitable. For another, when you're feeling those feelings of giddiness and everything that comes with new love for the first time when you're 18-20, it seems inevitable that you're going to think that it's more significant than it is. What's more is that you're going to be less trained in controlling yourself in that situation. If children are allowed to date in a more casual environment from an earlier age where they don't have nearly decade-old pent up sexual urges, it seems like they'd be more likely to have a healthy view of the life cycle of a relationship and not jump in with both feet just because they're horny and it's nice to be liked by another person.