Another comment, re depression in teens associated with sex. When I was a teen becoming sexually active, I regularly felt depressed following heterosex. I chalk this up to two reasons:
1) the very real phenomenon of sex leading to feelings of love (oxytocin, anyone?) that I felt were not reciprocated by my male partners. Boys, due to their biology, are often driven by their libidos, and were often less emotionally engaged. Or maybe they were simply less conversant with emotions, thanks to the society they were raised in. (My husband has introduced me to the male point of view on this. It must be sheer hell to be an adolescent boy in our society.) In any case, I felt like I was left high and dry, alone in endocrine turmoil.
So, emotional immaturity. I didn’t have the grounding to weather the roller coasters of giddiness and grief with grace. Who does, at that age?
2) sexual inexperience. Teenagers are just not good at sex yet. Boys are not good at pleasing girls, and most girls are not knowledgeable about their own pleasure. Neither sex places enough priority on female pleasure, in the face of the roaring biological mandate pressing on young men like a tsunami. (I still don’t see why young men aren’t taught to seek out middle-aged women as the first resort. Just you wait, teenage girls! You, too, will feel the urgency!)
Also, teens are often prone to depression anyway, even without sex. Sex can magnify whatever emotions one already feels. So I wouldn’t knock research that addresses this.