...while the situation appears to be unusual, based on the response here, I find the whole notion a bit refreshing. Before people flame me, I believe its been pointed out more than once that the OP wanted advice as it related to booking an extra person at the last minute. Granted, the whole side issue as the "why" was unnecessary...the issue was that "how."
However, as we all seem to be focusing on the "properness," I feel the need to weigh in.
*rolling up my sleeves and ducking slightly I'm impressed that Dad has not simply abdicated his responsibility at age 18. If your value system says its wrong to cohabitate (even for a vacation), then its wrong at 16, 18, 22, or 40. It wasn't that many years ago that the common conscience of society would have found the situation unacceptable, and I find it an interesting window on society not only how "Acceptable" it seems to be, but the level of distaste and (for some posters here) the anger shown here is nothing short of amazing.
The OP clearly is accepting of the situation, although disappointed. She clearly has the best perspective of the "why" and I think the rest of us should resist the urge to offer unrequested advice.