I think the problem is a lot of people are fearful of the idea that their teenagers might grow up into adults who enjoy "deviant" acts (they may be gay, they may have kinks, they may be promiscuous, or otherwise engage in behaviour outside of the generally accepted one-man-one-woman-ideally-in-marriage -PIV-only default).
And they find that shameful, and so want to portray more unorthodox sexual acts - like anal sex - as unhealthy, and misogynistic, when it's not the act that is unhealthy, misogynistic etc but rather the context of the relationship that act is being undertaken in. Sexual acts are neutral morally and whatever anyone chooses to get up to in the bedroom is completely ok as long as everyone involved is practicing safely, happy, and enthusiastically consenting. If there is pressure, if there is coercion, if there is abuse, THAT is what is wrong, not anal sex.
So they start to spread rumours that "children are being taught how to do anal sex and freaky sex positions in school" (which is not the case when it is properly looked at) in the hope that everyone realises how deviant and wrong the act of anal sex is and why anyone who does it is a danger to children and the people teaching this are after your kids. It's not completely dissimilar to narratives peddled about homosexuality some decades ago.
And while there is absolutely a balance to be struck between properly arming teenagers with information vs introducing ideas to children and pre-teens too early, I don't think a lot of reactions are coming from that place, even though ostensibly they say they are. Some of the language used on this thread betrays the underlying feelings.