16-year-old girls are not college age any more than 14-year-olds are driving age. Teenage girls are more likely to become sexually active with older guys and, in general, what the average 16-year-old girl is looking for from a member of the opposite sex she has just met and what the average 21-year-old male is looking for from a member of the opposite sex he has just met are very different and in terms of a power differential, the older male has the advantage.
I absolutely stand by my statement that this is an unsafe situation. Because sexual assault is so under reported most people are not in the position to know how common it is (particularly by "friends" and acquaintances). As someone who asks individuals about their history of sexual assault as part of my job, I am.
It is parents job to protect their kids and in this case the parents do not have the information they need to do that (or even to make their own call as to whether they're okay with this situation).
Honestly, I don't understand treating teenagers as if they are grown or nearly grown. Head scans of teengers resemble the brain scans of people who are brain injured. The areas of the brain governing insight and judgment are still half-baked until someone is 22 or so. The adolescent years are when kids have the opportunity to make decisions that have serious and long-reaching consequences. Leaving kids to their own devices at that point makes absolutely no sense to me.
Fourteen-year-olds can't legally drive. There's no such ban on 16-year-olds in college, it's not impossible, it happens.
Besides, it's a 17-year-old, and there are plenty of 17-year-olds in college. It's also her 16-17-year-old friends, and a bunch of '19-, 20-, and 21-year-old guys'
Which somehow became in your post, 16-year-old girls and 21-year-old guys. Which can coexist in college, but 17, 19, 20, etc., coexist in colleges alll over the place.
I don't buy men are rapists waiting to happen thing and that somehow a group of girls and a group of guys they know in a house together at the beach is 'unsafe' by its nature. Yes, I know there are plenty of sex crimes. I don't think that means any time there aren't parents around teenagers someone is getting attacked, nor do I think anyone needs a beach house to have sex.
As noted, the parents are renting the teens a beach house for a week's vacation. I don't know ANY parents, I can't even think of any parents I think I've ever met in life, who'd think that that'd mean there would not be any members of the opposite (or same) sex involved all week. They're going to the beach to hang out.
Hence, I think they've made whatever rules or had whatever discussions they're going to have and it's up to them. The 'information they need to protect' the teen girls they're again, renting a beach house for so they can go for a week, is that there might be boys involved? I really think this would be about as shocking as finding Lindsay Lohan in a ditch.
Girls - or boys - at a beach vacation, will find teens they're attracted to and hang out with them seems kind of obvious.
Regardless, I suppose we'll just agree to disagree. First, they are nearly grown, the people involved are, as I said, mostly college age (and almost college age) and are obviously together enough to handed a house for a week to go hang at the beach. I think kids learn better by doing than by being sheltered, personally, and it's not as if it's one innocent, helpless girl with a bunch of 21-year-old men. It's two groups of kids.
Same deal, it's the parents' deal to set whatever rules they feel like setting, or not. 
times a million! Also, the daughter is 17, not 14. At that point, she should have her own personal judgment to help her out. Is she going to college soon? Imagine, years of spending time with the opposite sex...
