Like I said, you have to draw the line someplace. If they were doing obscene things (and it would not be the first time this was reported) then as an adult I could ignore it personally, but kids are different.
There is an age; about 14 to 18, where kids are very impressionable. They want to try things they see, and they can become very defiant towards authority who tell them not to do something. Studies have shown that they don't really have brains well developed enough to understand the consequences of those actions.
I believe most parents these days do still try to maintain barriers for their kids. I mean do you really want to have to tell a three year old there is no Santa Claus? By the same token, should you be put in a situation where you have to explain to a 15 year old that engaging in promiscuous sex can be detrimental to your health and social well-being - even though there are two people 20 feet away doing it right in front of them?
Just for the record I just found this article on the problems LGBT people may encounter when cruising (it actually quotes CruiseMates - nice to see).
But it isn't just up to the cruise lines as to what they will allow on board - they always have to worry about what any one of these island nations may do if they find out there are certain laws being broken on these cruise ships.
The articles cites this incident which is veru enlightening:
This year, two gay American men traveling on a cruise were accused of having sexual intercourse on the ship while it was docked at Dominica. Police boarded the ship and arrested the men on suspicion of indecent exposure and "buggery," the local equivalent to sodomy on the island.
In an interview with Fridae Magazine, one of the men, Dennis Jay Mayer, said that the local authorities wanted to have the men medically examined to determine if they had participated in any sexual activity. Mayer also said that he and his partner were held in a cell for 19 hours with no light, water or toilet and they were taunted all night long.
After enduring humiliation and inhumane treatment, they were ordered to pay a fine of $4,000.
Now, naturally, I believe this is prejudicial and inhumane treatment of gay people, but one can't fight the law. My point is that cruise lines must worry about more laws of than just American, any cruise line has to respect the laws of every nation it visits.
Laws are continually changed and interpretations vary. Of course people can fight the law. There would be a lot fewer lawyers if not.
It is not reasonable to expect a cruise line to know the laws of every nation as they apply to the passengers and certainly not reasonable for them to enforce them, especially on a port by port basis. That would be absurd. I suspect it is not legal for them to discriminate.