To him the 'facts' are that gay relationships and marriage equality are sinful, as is abortion. If schools teach anything differently then to him this will not be 'factual', but 'indoctrination'.
What are you basing that opinion on? Robertson's blog says "I gave my considered opinion and whilst I do not expect every one to agree with me, I do expect that in a democracy my views are entitled to be heard without being subject to the kind of irrational and prejudiced abuse demonstrated in the article. "
Robertson also said other people who do not share his beliefs, including his beliefs on SSM, are free to choose to get their rules and principles from sources other than the Bible. So based on his blog, it appears that his concern is about indoctrination of children with the beliefs, rules and principles from other sources as opposed to facts.
Morals and ethics about SSM and bullying are not facts. That SSM exists in society and that children at school might be from diverse family backgrounds is a fact. That bullying people because they seem different from the majority - whether it is because of race or to whom they are naturally attracted or their religious beliefs or their atheism or their moral or ethical beliefs or how they self-identify in some other way - can cause harm is also a fact.
I say "can cause harm" as there are many children who can shrug it off. I had kids spit in my face and call me a Paki at school during the 70s and some adults were less welcoming of my brown face or cultural differences but it just seemed part of the rough and tumble of navigating Life. Lots of people were nice to me too so I didn't dwell on the ones who weren't and just made street-wise choices about where I walked or went by myself in the playground and park and walking home from school.