Sooner or later they are going to come into contact with such material.
Correct. This is as it should be. Young kids are simply not equipped to deal with some of this. It can cause real trauma. As kids get closer to adulthood, parents with good judgement will relax the controls as they see their kids' judgement developing.
If this material is forbidden it will also be more attractive
So this isn't any of your business, regardless of how true your statement may be. Parents get to parent how they see fit, within parameters society defines[1]. Lots of parents do all sorts of things to their kids, both gross and subtle, that I think is awful, damaging, ineffective or stupid. But that's none of my business, either.
surely its better that when they first encounter such things they do so under the guidance of responsible adults who can explain what it is.
Yes, absolutely! And part of that is judging when they're emotionally mature and intellectually developed enough for a given topic. Look at it like this - trying to explain military strategy to a four year old is doomed. There is no point in trying to explain it someone who doesn't even really grasp the concept of societies, let alone why they might fight. Explaining BDSM to someone who doesn't understand any relationship modes other than parent-child is similarly doomed.
Thus, a parent may want to try to delay some of those conversations.
[1] I make no claims about the wisdom of the specific rules in any given place, just describing.