You think? You think the leaders say "let's restrict sexual activities so we can better control the masses."
No, I think it is instinctual. And, once again, the leaders believe these things themselves. But look at how the pattern repeats itself, over and over, in religions that end up controlling quite a bit of the followers' lives. They exert some sort of control over sexuality as one of its primary tenets. Sometimes the control is manifest by demanding expressed sexuality, but in a certain way, or with certain people. I don't think this is a coincidence.
IF all religion is made up, then what you say makes sense. I find reading it in that light much more realistic than those who believe that the first presidency and apostles are all in cahoots to make us their slaves. If indeed religion is made up, then it seems natural that religions most effective at getting followers to act in a particular way would the ones to survive the longest.
No, it's not "cahoots", like some sort of conscious conspiracy. The leaders believe it, too. Your last sentence is closer to my thoughts, that this is part of an evolutionary process. I believe religion serves an evolutionary purpose because it is universal, and the most likely reason is the binding power for the group. It's the ultimate in tribalization.
All that said, if making masturbation a sin is a big thing for teenage boys, how do they keep teenage girls or indeed adults in check? It seems that at this stage in life control over my sexuality is done more with my spouse than by the church.
First, teenage girls are not asexual, but certainly control over sexual feelings is not the only way leaders control followers. And as an adult Mormon, you still are under constraints.
Take another intimate example - the wearing of garments. Why wear garments? So you are constantly reminded of your religious beliefs and obligations. Having to control sexuality in a certain way is another constant reminder.