Well, I am not sure where you got your information, but I went on Singlescruise.com Halloween cruise on the Glory and we had a blast and the only ugly people I saw were on Halloween night and they were working towards that goal.
Yeah, there were plenty of young, good looking 20-30 yr olds, and many of the 40 to 50 yr olds weren't too bad looking either. I know that, as in real life, there were some average looking people in the group, but we allowed them to stay, since they had such great personalities. There were a few pathetic people in that bunch, but that was their attitude that made them unattractive and we had a good time inspite of them.
I suppose if you want party with only the beautiful people you will need to get your own yacht, but until then use a bit of common sense and maintain an open mind and you will have a good time!