A few issues with this...
The concept is vague: it leaves open to interpretation exactly what constitutes discussion of religion, sexuality or sexual orientation. Who decides what is "explicit"? And likewise can teachers really be expected to remember exactly what is explicit and what is not? Won't that create the tendency to stay away from the vague middle ground in fear of legal action being taken against them?
Information control: allows parents to decide what their children may or may not know about; it is assumed that what is taught in the present day education system is sufficiently objective to justify what is discussed as non-ideological. (Even though the choice of specific texts within the curriculum may be more or less relevant, depending on your outlook/background.)
Human rights: as above, the ability of parents to control information affects the child's ability to make choices (which is vital in a democratic system).
This sets a precedent: bending to pressure (in this case from religious groups) opens the door to similar changes down the road (i.e. the slippery slope).
Granted, one could argue that if the situation were reversed and the state education system taught let's say creationism (or something that rejects evolution theory), that more progressive parents would also be asking for the right to keep their children from being exposed to that dogma.
Thing is, education in the present day western world bases itself heavily on empirical principles (i.e. what you can prove, not what you cannot). Thus far, the evolution theory, for example has not been effectively disproven, nor has any better theory become available (i.e. theory of evolution is based on empirical research and also follows strongly from logic--so most of the intelligencia in the western world accept).
Sexuality is a bit different because AFAIK, it has not been proven that traditional religious-based sexual norms are less effective at preventing unwanted pregnancy for example than sex ed. classes.
Sexual orientation is pretty much the same story as far as personal safety is concerned, but the issue of human rights is more complex (i.e. to allow children to be shielded from something that is considered to be acceptable in this country--thus in effect making it seem wrong--could be unconstitutional).
There's also the fact that these religious sectors of the population do vote and have a right to have their views represented. Then you run into the problem of having to balance the interests of the population (i.e. what happens if children are too different in terms of knowledge when they grow up--already an issue with private schools--and this adversely affects the well-being of all in the state) with the interests of the few (i.e. a parent's right to protect their children from any harm, even from the state's influence).
As for the inclusiveness aspect: the last thing you want is to alienate these groups because you will only be making it worse as far as the children's education is concerned. That said, the state also can't be bending over backwards for every group that has some issue with the curriculum (esp. if the purpose of education is to create a skilled--knowledgeable--labour force and not to impose some state ideology on the population; with the additional requirement that the populations of democracies should be objectively informed in order for the system to function effectively).
IMO it's not really a question of religion vs. secularism but rather a question of the freedom to make an informed choice vs. freedom to protect those who are highly susceptible to influence.
