The school needed to apologize for the presentation - sexuality instruction should all be PRE-APPROVED by the parents. The sister should also have changed her presentation when/if she realized that parental consent was not granted.
I could care less if she gave teaching authentic to the Magesterium. I don’t discuss Just War theology with my 3 year old and many parents don’t want sexuality discussed without their consent. Premature sexualization is EVIL - and for some of the more conservative members of that high school this was premature.
She wasn't talking about specific homosexual sexual practices, as in Massachusetts public high schools. She was addressing the evil of homosexuality itself, and its association with other social pathologies.
She was speaking to a high school audience at a Catholic school. It would be no different from speaking about the fate of Sodom and Gomorrah, or about adultery and divorce.
The parents were objecting to the criticism of homosexuality per se.
So your hypothesis is incorrect in the light of the arguments made by the anti-Sister-Jane petition (which you can still google, I think, using the search words "Emma Winters" and "Charlotte Catholic High School").