marrying with the primary focus of just to have sex is a terrible idea and a good way to end up with a failed marriage
This. I believe in teaching to respect yourself as well as the other person in the choice of sex. You should say no because you respect yourself, and your boy/girlfriend, in relations to what you two are ready for (consequences as well as pleasure) NOT because it is sinful. Teens need to be taught, first and foremost, how to communicate openly and honestly with those they love and have feelings for, along with their respect for God.
I've said this before, but teaching sex as the prime goodie of marriage is counterproductive. Because no relationship can live on sex. When you teach it is the reward of marriage, you imply it as the pillar for which the marriage stands or falls. You set up an idealistic fantasy in the minds of young people when you paint it as this ultra-magical experience. That's why porn addictions wreck relationships - the films causes the man to lose touch with what real sex is, and thus is not as satisfied by his wife. Porn is wrong, imo, for the same reason such forbiddance of sex is wrong - it creates unrealistic expectations of the act itself when it comes - ironically, while also painting it as lewd and horribly wrong at the same time in the present.
There may be a point where one partner CAN'T have sex, for psychological or physiological reasons. I'm not saying it's taught that sex is the only reason to get married, but it is withheld, WHILE EVERYTHING ELSE, MORE important emotional benefits of a marriage, is allowed. You can share yourself with someone in every way but residence and body - how does that not condition one to think that is all that holds the marriage together, since it is only allowed for marriage? And how is something that is wrong made right because of a piece of paper? You can get baptized and it be just "getting wet" if your heart is not right, but every marriage certificate is written by the finger of God regardless of the heart? In the eyes of the world, you are married, and that is required by the Church, to regard one married as the world does. Using the world's standards to define the institution. What?
Because a man decided we were worthy of marriage? This is another reason teaching strict forbiddance is dangerous - if you spend the better part of your youth, say until you're 30, resistancing sex under the idea it is sin, that mindset is probably NOT going to magically disappear when you're married. Your default is still "this is wrong" because that's the only way you ever thought of it. Many couples struggle to satisfy one another, and because they often enter into it with ignorance and naivety of the subject, that frustration is often fatal to the marriage, along with other life pressures... one reason sex is really good for a marriage because it is a release, and helps the couple relax and bond - it is psychological, as much as it is physical. This is NOT to say that unbelievers are soooo much better at marriage and selection of spouse.
I've said this before: I think it's great if one is lead by God to wait, meets a great man/woman, and has a wonderful marriage. I am not discouraging that, and I think it is ideal and good for a clean slate in terms of the wedding bed... however, if you write on that clean slate for a few decades with "sin" "shame" "unholy" "carnal" then it is going to be hard to wipe away. When couples do find they are having intimacy trouble, it is still thought taboo oftentimes for them to get some kind of sex aids, or even sex counseling - because that's not leaning on God, you see. So, couples are often restricted within the marriage itself how to fix it and please one another, because the act itself has certain possibilities that are "perverse" even between monogamous, heretosexual married couples.
I submit there are some Christian circles that are not near this prudish, and advocate things a person can do before marriage to be ready for intimacy, while maintaining sex for marriage alone. So I know not all circles are this strict.