My $.02 is that the boy PROBABLY should not come but for reasons other than most posters are giving. I am of the school of thought that teenagers should know that their parents trust them UNLESS they have given them a reason not to. My own parents let me go away with my high school boyfriend for a weekend with his parents. We went to a waterpark and had a blast! There's a couple of circumstances where I would consider allowing my daughter to have her boyfriend along:
1. It was a larger group and DD was the "odd one out" in terms of age and interests. If I had very small children and DD was much older then them, having the boyfriend along would make the trip enjoyable for her where it would otherwise have been boring... then yes, I could see letting him come along.
2. Sleeping arrangements would have to be such that I would not be sharing a room with the boy. I agree with another poster who said it would definately detract from the vacation. There would have to be more room and DD would sleep in a room with me and the boy could sleep elsewhere. (I do think that letting them sleep in the same bed is TOO MUCH trust and would leave the impression that you were condoning sexual intimacy. There's trust and then there's giving them a free for all! You still need to show them that there are LIMITS to what you will tolerate or condone.)
I don't think that a parent should be psycho about keeping their kids away from the opposite sex,though. My boyfriend was allowed in my room as a teenager. We were allowed to lay on my bed and watch movies/TV. The door was never allowed to be completely closed. We NEVER thought about doing anything! (We saved our making out for the back of his parents minivan.)
If you have a child who is trustworthy, then keeping them from spending a whole week with their boyfriend seems like a missed opportunity for them to see what it means to REALLY be intimate. And by this I mean giving them a glimpse of what it's like to spend nearly every waking moment with someone. It teaches them how important compatibility is. I don't think 15 is too young to learn this. I would think it would help them to make wiser decisions about who they select in the future (assuming that a relationship between 15 year olds would never last).
But what you ultimately have to ask yourself is... can you AFFORD to turn an expensive vacation like this into a "learning experience"? Do you go to Disney often enough that the trip wont be a really unique experience? Or is this a once in a lifetime/rare trip for you and your DD? Also, it sounds to me like you have an opportunity here to spend some quality one on one time with your DD and she doesn't need to have someone else there to keep her company on the thrill rides and such. You're fully available to her and that's all she needs.
I'd explain to her that your reason for saying no is not because you think she's "too young" or that you don't trust her. Those kinds of excuses, be they true or not, are not going to bode well with a teenager who needs to know that you trust and respect her. But if you tell her that you want this to be a special trip for the two of you because she's getting older and you want this memory of just the two of you, then she'll see that your decision is based on more than just a kneejerk reaction of thinking the trip is going to result in unwanted pregnancy or something horrible.
One of the greatest things my parents ever did for our relationship was to give me their trust. By not assuming that I was going to do terrible things, they showed me that I was a WORTHY human being who was capable of making wise choices. And because of this, I NEVER wanted to disappoint them. They never made me feel smothered so I felt no need to rebel.
But each child is different and only you know what your daughter is capable of and where her heart really lies.
but in this world of boy/girl sleepovers and more relaxed attitudes of boy/girl interaction I wanted to make sure that I wasn't off base. I was raised by my grandparents and sometimes feel that I'm already "off" a generation in my views.
Looks like it's going to be DD15 and myself for a ladies only trip.
Yeah, right!!!!! That certainly didn't happen. 
) and now if it's worth it or not.
