"The reason there is no conversation is that they don't consider there to be any costs to their kids being unable to have sexual relationships with adults."

I have observed a similar phenomenon in religious groups - they advocate a certain lifestyle that they claim would be better, but none are good enough to live it themselves.

But if someone should manage to live according to their edicts - they will find that this lifestyle is not better at all, but worse. The religious group manages to function because of their failure to live their dogma. Their religious beliefs often act to moderate their behavior - to prevent extremism that would be even more harmful - but they never see the actual results of living according to their ideals because they never actually manage to do so.

So when they teach their kids to avoid all sexual relationships, they may know that they themselves did not abstain entirely. Perhaps they never "did the deed" but they snuck off and played doctor, practiced kissing, and a dozen other things - whether with other children or adults - but they presume that they were the exception that was not deeply hurt by this, and that their children would be better off to avoid these entirely - never acknowledging the good effects these things had on their own life.

Now, it gets a little more complex when you add the disclaimer "with adults" - much of the good from these explorations will happen whether their partners were above or below the age of 18 (16 in Europe). On the other hand, other types of relationships with adults will certainly have an effect on how children turn out - and we are only now beginning to see a generation of adults who were almost entirely cut off from older generations while they were children. Perhaps we will see the results of that sooner than we would like, in a generation that has no roots.