I think a lot of the anger is the air of authority about this one.
It's promoted by the curriculum, by the Healthy Ireland section of the Library. Trustworthy institutions. Its not some random "dirty book" that teens might be drawn to, and somehow get their hands on.
It's the suggestion that casual hook ups with randoms is the norm and that you must be proficient at sexual skills to please said randoms - most people still believe that first love and learning and exploring sexuality together as peers is the best way to learn sexuality.
If you're already anti establishment, or pissed off with the government it's just another data point that they're out of touch, or imposing their values on people who simply don't see things that way.
And it is true that a lot of LGBT friendly policy is being put through under the radar. People are noticing the extent of it now and noticing how it impacts other groups. It's OK to oppose policy if you disagree. Even the far right/left extremists are allowed to protest. That's the essence of liberalism.