Regardless of whether they identify as conservative or liberal, those who think that children should be shielded from hearing any sort of reference* to LGBTQI people or relationships are expressing discriminatory beliefs. Homophobia and transphobia are the terms applied to such discrimination.
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* By “reference”, I mean the kinds of things that a straight teacher would talk about with their students (e.g., their spouse, their own children, etc.). I do not mean anything that can be considered “indoctrination” (spoiler: you can’t turn kids gay or trans).
ETA: I do not believe that those who are homophobic or transphobic are automatically bad people who can’t unlearn their bigotry. My parents were homophobes before learning that their son is gay. Even I had long-internalised homophobia that I had to work many years to get rid of.