If people in same sex relationships exist and are an accepted part of society (which I assume you have no problem with, but do correct me if I'm wrong), then children do need to have this explained to them. Especially if they come from families who aren't likely to explain this to them at home.
As to your question about family arrangements - it could well be the case that the traditional family is on average better for children. As far as I'm aware, the available evidence indicates that there is little significant difference. But let's assume for the sake of argument that's all wrong or the result of new world order liberal conspiracies or whatever. What exactly is the implication you're making about what should be done? Are we really going to forbid all family arrangements that are even slightly sub-optimal? Shall we leave children in care because the perfect family is not available to them? There's about 70,000 children in the care system last I checked, and adoption rates are falling. Even if (if!) same-sex parenting is sub-optimal in some way, it seems like bad policy to reduce the number of potential adopters in such circumstances. Not to mention the fact that it would take away rights from same-sex couples that heterosexual ones have and is therefore fundamentally against the principle of equality.
If you don't think we should be disallowing same-sex couples the same rights as heterosexual ones to adopt children (and why should we?), then we're back to square one when it comes to education. Same-sex relationships, and same-sex parents, are part of society, regardless of whether you or these religious lunatics like it or not. This needs explaining in an age-appropriate way to children if our education system is going to be worthy of the name.
As for your assertion (and assertion it remains) that this is all a plot to undermine the traditional family - I'm afraid you'll have to prove it. People who actually, genuinely, do want to undermine the traditional family typically hate this sort of thing (gay marriage and gay adoption both, and teaching kids about it) because they think it robs homosexuality of its dissident nature and makes gay relationships "respectable" when what they actually want gay people to do is be living critiques of "bourgeois" respectability itself. People like Ryan Conrad are particularly influential on this, but he's by no means alone. That's what radical leftists actually think, as opposed to the cartoon ones you have imagined masterminding this policy.