I think you are conflating teaching with discussion. There is no "sexual orientation 101" class. However, LGBT people exist and therefore students might have classmates that raise questions, or the class might want to read a book that features a gay character. Under "don't say gay" laws, none of this would be allowed to be discussed. Neither would many historical topics revolving around civil rights. This severely limits perspectives and harms teaching possibilities, not to mention removes valuable resources from children that might need it the most.

Also, I learned about many religions in public school. School shouldn't tell you which way is the "right" way to be, but it should educate students that there are many people of many different religions out there, and explore the historical and cultural significance of them. Discussion of LGBT topics should be the exact same way.