Someone telling teenagers they can't have sex, whether that someone is the law, their parents, society, or anyone else, is not going to stop them from doing it. What it will do, however, is make them better at not being caught doing it.
Trying to prevent teenagers from having sex will simply breed societies culture of shame around sex. If your teenagr child has caught an STI, and they've done this whilst you've been constantly telling them not to have sex, they're not going to talk to you about it. They're going to keep it to themselves, and simply let the problem grow and grow, until they're eventually rushed to hospital with life threatening problems that could've been dealt with by a few antibiotics if they'd spoken to you about it.
Point being, teenagers are going to have unsafe sex and you can't stop them. However, you can help them to deal with the consequences by not constantly telling them that they shouldn't be doing it.
Teenagers are randy stupid little rascals. They think they know everything and that they'er the center of the universe.
The more you try to tell a teenager what not to do that's all they want to do. The only way is to let them make their mistakes and be there for them. They will make mistakes, we all did. And still do.
Also some people mature at different rates. Some teens will be sexually switched on while others will still be scared of talking to a member of the opposite sex.
It is also at that age we begin to understand our adult selves for the first time. Never again in life will a person be in that sort of environment where there are so many open minded young people. It's a special time in life.
I do believe it's sad that our society encourages young people to be fickle and vain as these traits do not serve us well in life. However we grow and learn.
While some may regret it, the majority will not be too bothered by it. Forbidding them to engage in sexual relations with each other (as 2 teenagers) would just make them want to do it more. Our best bet, and the way to make it the least likely to have problems is this: Teach them how to be careful, warn them about all the things that can go wrong, and do not demonize the act, do not make it seem like it's the worst thing they could ever do (because they may go do it even simply out of spite when you "ruin their whole world" which apparently happens a lot). The best thing we can do for teenagers is help them NOT be ignorant, they need to know what to expect, to always be prepared. They need to know the risks and how to defend themselves from them. Prepare them so that they will not be at risk when they inevitably do something stupid. Yes there are exceptions but shielding every teenager from it, and making it that unholy sin which will make them burn in hell, not only won't stop them, but if it does it will disturb future relationships they will be in when they are adults and can make their own decisions.
Intercourse is a special thing that should happen between a husband and wife to solidify their marriage. Progressive society has made sex into an object of transaction, where two people just exchange things for the benefit of the other. Sex should be kept more sacred for partners, and not be an event you do anytime you want to with whomever you want to do it with.
To any who don't like that argument, allowing underage sex is just going to increase the amount of underage pregnancies and STDs. And it is just not healthy for the minds of youth. Stay in school and study.
When they are indulged, this unknowingly affects their emotions, that innocence is gone and they will always be distracted and stressed out mentally and this derails them from the once utmost focus and enthusiasm they had for life. Teenagers do not have the mental capacity ton handle the stress in sexual engagement and this experience may turn them to sexual addicts and perverts which is an obsession that can never be satisfied. This reasons are devoid the health threats they stand to face due to ignorance or inexperience or untimely exposure.