Why the "it's a private institution and you know what you're getting into when you attend" argument doesn't fly with me in re: BYU

In the first place, no, you don't. Average eighteen-year-olds, especially if they have not grown up in Utah, do not know everything the Honor Code entails when they decide to go to BYU. All they know is that BYU is their church's school. "The Lord's University." That their parents and/or leaders went there, that they are encouraged to go there and praised/thought highly of for doing so. That BYU is a school where they will be able to have an educational experience like none other, accompanied by the spirit in every class. They have great incentive to go there. But that doesn't mean they know what they're getting into.

They certainly don't know that they are going to be encouraged to watch their roommates, neighbors, and friends, and to report those who don't follow the Honor Code to the letter. They don't know that if they are struggling with their testimony and confess their doubts to their bishop, they might just get kicked out. They probably don't know that members of the opposite sex will not be allowed to use the bathroom in their apartments except "when civility dictates", and since this is a completely subjective guideline their ability to extend that civility will be dependent on their roommates' cooperation.

They don't know that they might be sitting on campus minding their own business and have a fellow student hand them a note castigating them for being attractive to others, or that they might try to take a test and be turned away because the testing center employee finds fault with their dress even though it's modest in every way including the actual meaning of that word. (Many of them probably do at least know that they can't take a test with any scruff, much less a beard, but even then—not all.)

In the second place, it doesn't matter whether or not students know what they're getting into. The point is that this school is a church institution. What happens there is either officially sanctioned or unofficially justified by the fact that the school is governed by the leadership of the church (the president of the university is a General Authority, and the Board of Trustees consists of the First Presidency, the Twelve Apostles, the Relief Society and Young Women general presidents, and the senior president of the Seventy. Our prophet is the chairman of the board). If there's something that outrages people, something that shouldn't be happening, then the fact that it's a private institution actually makes it worse. That says, "Yes, the LDS church runs this school. And this is what they want to have happen there." 

"If you don't like it, don't go"—this means nothing. If you don't like something your church is doing, does the fact that it is not personally affecting you make a difference? Does the fact that you had the choice whether or not to attend the school actually change anything? When it's your religion involved, it's kind of a matter of principle. 

I'm not saying this to say that I think the church should be held responsible for misguided patriarchal control freaks like the Valentine's Day Vigilante. I don't. All I'm saying is that, even without addressing the fact that for members of the church—who are pressed and pushed and very strongly encouraged to go to college—BYU is an absurdly cheap option next to comparable schools and therefore everyone who doesn't have a lot of money might not have as free a choice as its proponents like to think, this argument doesn't hold water. It doesn't matter that people have the choice of whether or not to attend, and since this point is more often than not just an attempt to dismiss people's concerns by questioning their right to have them in the first place, we'd all be much better off if it could just go somewhere and die out peacefully.