Are you saying it's an irrational taboo? Or a rational taboo? Is using 18 as the magic age for buying porn really the best we can do?
The problem with rational decision-making as a universal strategy is that people can be really bad at it. Especially when they really want to hit on that hot 14-year-old, or stab their bastard half-brother, or eat that triple cheeseburger. You'd be amazed at the kind of "logic" that people can apply in a situation like that. Or when the data that's required to make a rational decision is outside their personal experience. (If every generation had to experiment with living in the swamp -- losing half its population to malaria in the process -- it would be a lot less efficient than a simple taboo against swamp dwelling.)
So, yeah, it's irrational. Sometimes that is a good thing. More often you need to use some applied hypocrisy to keep your taboos aligned with reality. And sometimes it's time to chuck the old taboos and develop some shiny new ones.
In the particular case of 14 year olds and porn DVDs? Technology is moving so fast that the taboos can't keep up. Only a few decades ago it was taboo for fifty-year-olds to rent porn DVDs -- read Eric Schlosser's Reefer Madness for examples. That is changing very, very quickly thanks to the Internet, which has eroded the porn taboo pretty thoroughly. Who cares about DVDs anymore? We've only just woken up to discover that every 14-year-old in America has a free porn machine sitting on his or her desk, and social norms are struggling to catch up with that.
When trying to explain why the cultural taboo exists, I'm not sure you get to elide that problem. How do you "avoid" these dangers? Societies try to do so by inventing cultural taboos!
Sex is inappropriate for children because (society presumes) most of them are not equipped to say no and make it stick. They lack the emotional, financial, and legal resources to "break up" with abusive or manipulative lovers -- particularly if those lovers are adults, and most particularly if those adults are family members.
(Note that, by this logic, sex is also inappropriate for a lot of adults. And there might be some people who are legally "children" who are nonetheless responsible and wise enough for sex (I certainly wasn't one, and I can't say that I've ever seen any, but it is theoretically possible... :) Like many societies, American society tries to deal with this ambiguity by defining a magic calendar age at which children legally and morally become adults. Alhough nobody ever said that this was a particularly realistic model, it's the one we've got. We do tinker with it occasionally, on a case-by-case basis.)
Why sex and not, say, football? (It's a good question -- a lot of kids get hurt when they're pressured into playing football.) Probably because sex carries a particularly heavy set of practical, social, and emotional consequences that are difficult to separate from it. (Birth control helps a lot, but it is not enough, as many AIDS patients and stalking victims know.)