Less teenage pregnancy is good because teenage pregnancy is associated with poor outcomes for the children and parents in that environment.
Sex is pretty wonderful for your health and metal well-being, if you minimize the threat of STIs and pregnancy.
Education and programs that allow the under-aged access to healthcare facilities that specialize in sexual health and family planning seems to be the more healthy (and more sustainable and achievable) alternative to abstinence advocacy.
I think that anyone that believes that teenage abstinence is at all achievable is probably delusional, or at the very least heavily sheltered, on the topic.
Seems like if you say the wrong thing about theocracy, or have sane and body-positive sex advice, you're bound to get some silent judgement from people who aren't brave enough to actually call you out, lest they have to actually defend their beliefs that cause them some degree of cognitive dissonance. It's likely easier for them to not question why they think the way they do, and if it's the right way for them to think/react/etc. at all, so those beliefs and assumptions go unchallenged.
Obviously this is all speculation and assumption, but if someone won't engage with you to explain themselves, that's all one can really do, for better or worse.
I'll make an exception for you though, since you may have a misunderstanding. You mention "theocracy". Atheists can believe that sex should wait for marriage. No religion is required to hold that belief. It's the same with "body-positive". Abstinence until marriage doesn't mean hating bodies or sex, and it doesn't mean being ashamed of either.
All it takes is a preference for low-risk orderly lives without all the tragedy that sex-related mistakes can bring. Since behavior affects others, including sex partners and offspring, there is a duty to avoid giving them an increased risk of tragedy. Breaking hearts isn't nice at all, and that gets far worse when sex is involved. People usually consider the first time to be special, and it would be best to share that memory with a spouse. It's awful to think that a person would be longing for a previous partner; having a comparison could weaken the current relationship. Moving from one partner to another, repeatedly breaking the emotional bond, weakens the bond that can form.
If presented with a nation (comparable in HDI to the US) with average age of first sexual encounter for women being 21+ would your posterior probability of teenage abstinence being achievable be different from your prior?