It seems perfectly reasonable to say that people should just stay away from BYU if they don't like the rules and lifestyle. However things become a little bit more complicated when other incentives are involved.
For example, if the student in question didn't get into any other comparable academic schools (I know with BYU that's a long shot, but for the sake of argument) or if he was offered an athletic scholarship that made college affordable when it would have otherwise been out of reach -- in those cases the student would be forced to choose between his education and rules that most of civil society would find more than a little bit unreasonable.
It is more than a little bit like the army telling young men of few means that their only way to an education is if they sign up to commit yet-unknown atrocities. Sure they're able to quit at any time and forfeit the education, but doesnt that seem like a compromising situation to be in from the start?
I think that we can all agree that nobody should be denied an education because of their completely natural human urges and that education itself is about teaching, not issuing mandates that fly in the face of human nature.
I can see it now:
"Please come and run for us... We'll give you a full ride!"
"Great!"
"Only don't have sex...or wear bright colors...or eat ethnic food."
"Huh?"
"Yep. Those are the rules we made up. Sex is bad for you."
"That's a bit strage. I don't remember hearing about that."
"Well, you just did. Welcome. Have fun. Only don't have too much fun or fall in love or else we'll kick you out."