> I think they're suggesting that young children--in their critical (mental) periods--can somehow turn gay or transgendered due to exposure/normalisation of gay and trans people.

My personal belief is that the exposure/normalization is good because it helps people discover themselves. And I read that comment somewhat differently: exposure to these concepts might be controversial because parents have such different impressions of what is "right", so they might end up accidentally brainwashing their child. It already happens a lot, and people obviously don't want more of it.