I looked at the studies you cite. I don't think you can draw the conclusion that adolescents "aren't interested in sexual activity of any nature" from a study based on self-reported frequency of masturbation or sex. The decline in masturbation could be a real one, or it could be due to either today's youth being more reticent to admit to having sex or masturbating, or the previous generation exaggerating their sexual activity. And if there was a decline in masturbation, I don't think you could necessarily say it's the result of adolescents being uninterested in sex, given the study didn't ask WHY the people self reported lower rates of sexual activity.

Finally - a reduction in school aged people having sex is not a bad thing. Having sex in high school doesn't have happen to develop into a normal sexual adult. In fact, it could be a positive sign, showing that adolescents are willing to wait for what they actually want rather than be pressured into sex, Euphoria-style.

I didn't have sex until I was 23. Not because I wasn't interested in sex. I had and continue to have a raging libido. But I wanted to wait to have sex with someone I loved and trusted. Because I didn't want to be used or be objectified by someone who only cared about my body and not my personality. And I was very happy with my decision.

Now I did masturbate a lot and still do. But I don't think that masturbation rates have really gone down. It's not like porn site traffic has declined after all. If anything I'd say the studies were flawed.