It adds greatly to the bond between the people in question, and brings them together. This is all rooted in our evolution. Our brains want us to find good partners, want us to mate and mate often. Some people require several encounters before they feel comfortable sharing that part of themselves, others may feel comfortable upon first meeting, and for others, comfort is less of an issue and they don't require the same kind of emotional bond in order to enjoy it. Again, being smart about how you do it, drastically reduces your risk of finding yourself in a spot. It's never zero, but few things are.
How am I going to accidentally going to have a baby? Do I have less ability to prevent a pregnancy than I do in preventing a drunk driver from running a red light? With the pregnancy, a couple can use a condom (with proper condom use, an average of two pregnancies result from every 8300 acts of sex), use oral contraceptives (99.9% effective), use a combination of the two methods, or limit sexual activity to oral and anal intercourse, any and all of which reduce the chance of pregnancy to nearly zero.
And what about gay couples? What about my friends who are in their 70s? You must agree that any reasonably educated person has much more ability to control whether or not they conceive a child through their own volition, than they do to avoid a potentially life-threatening car accident.
But not everybody who has sex is always doing it with the person they want to partner with for life. Yes, there are risks involved, but the aforementioned precautions reduce that risk considerably.
Do you only get in a car or on a public motor way to go from your residence to work and back? Or do you risk your safety to leave your house to go running, have a meal with friends, see a film, etc.? Somehow, you decided that it was worth the risk to do those things, and in that vein, you try to avoid foolish actions that would increase the risk to your personal safety (mobile phone use while driving, not checking for traffic before crossing a road, drinking while driving, etc.). And despite all that, you could end up like that family that had a tow truck land on their car.
Similarly, people can have sex with all the precautions listed above. Most who take those precautions won't end up with an unintended pregnancy or an STD. Those that are more cavalier, like those who drink and drive, text while driving and run in the middle of the street, put themselves at greater risk.
How would you suppose a couple that has decided they don't want children will accidentally have some? If Carl has had a vasectomy and Carla has had a tubal ligation, how are they going to accidentally end up with children? This is what I mean by taking proper precautions.
The point is the risk is not zero.
Practicality would mean to extent to which an idea can be realized in the real world, the likelihood of that idea to succeed when implemented, and the suitability of a solution for a given situation (contrast this with your initial post on this thread, countering that adolescent sexuality is not normal, "Rationalize whatever you want, but it's still wrong.").
Given that a majority of people have sex before their first marriage, and given that a majority of people do not contract STDs or have unwanted pregnancies, it would seem that having sex (with the aforementioned precautions) is a practical solution in satisfying the urge to engage in sex with your partner (short or long term).
A 16 year old committing to a lifelong marriage is not necessarily reflective of their preparedness for anything. Can you see why most would think that the person with decades more behind them would know themselves better, have more life experience, be more mature and be better prepared for such a commitment.
Would you honestly say that a 13 year old wife (perfectly legal in the state of New Hampshire) is more likely to be prepared to handle sex and her marriage than a a 40 year old who has graduated middle school, high school, university and has spent years getting her career in order, simply by virtue of the fact that the 13 year old signed a marriage certificate and said, "I do"?