I would use the term expression of sexuality rather than romantic expression as I don't believe the term romantic expression has as concrete a meaning as expression of sexuality. Nonetheless I do not see the need to teach children sexuality. What about sexuality does a child need to learn from a teacher that the child is not already aware of?
I do not see how pointing out to children that, though some people have different sexual attractions all but one or two such attractions are equally valid expressions of sexuality is anything other than engaging in indoctrination. The former i.e. that there are different attractions is a fact. The later i.e. that one type of attraction is as morally valid as another is an opinion. An opinion that few adhere to when it comes to certain variations ( pedophilia for instance). Teaching children that a particular opinion is the correct one to hold would be IMO indoctrination. I contend that we all indoctrinate children and not only that, but there is good reason for us to do so. It is foolish not to make sure that our children are indoctrinated in the values we ourselves hold. If we do not do so, then others will indoctrinate them with values we are opposed to. Indoctrination into a value system does not preclude a thinking adult from reaching different conclusions on their own but indoctrination coupled with being deprived of the tools for critical thinking will do that. I prefer the education system to fight indoctrination by providing those tools rather than using opposing view indoctrination for that purpose. If a child is being indoctrinated in opposite directions from two sources then the child must pick a side and see the one side as ally and the other as the enemy. If instead a child is taught how to think critically as the child grows he/she will mature intellectually and realize that she/he needs to examine what he/she has been told and be able to decide rationally what to believe and not believe. In this way the person does not see the world as the good guys vs the bad guys but as humans with different ideas that are all neither all good or all bad but just trying to figure things out the best way they can. They will not see people with different ideas about sexual morality as having evil intent but just as people that have a different POV. The "I'm right and you are wrong on purpose because you are evil" mentality does not exist for a critical thinking person.
Romantic entails expressions of affection in the specific relationships where the love is of that exclusive nature and not friendship or the like, it's not that complicated or unclear. And I didn't say a child needed to learn about sexuality, that's arguably for teenagers in health and wellness class. A child can learn about how you should ask someone if you want to hug them, how kissing has particular implications depending on how you do it, etc. I believe the Netherlands has something in that vein where it's quite age appropriate and yet teaching kids about such things like consent and affection in a way they can understand
Except when you're pointing it out and showing how there aren't really any major differences, the condemnation of one being unnatural, etc is purely preferential and subjective in the first place. It's not like kids can't learn about evolution and then have their parents indoctrinate them horribly into believing creationism and the like. Having the knowledge is not the same as saying the knowledge is unquestionable, that's religion's purview
Nice try, but not all attractions are equal in terms of the behavior that results from them, pedophilia being demonstrably harmful, because children cannot give informed consent. If the difference is superficial at best, like a man liking a woman versus a man or a woman liking the same sex, then it's not the government's job to talk about prescriptive ideas, it's still just saying that these exist and that there's not necessarily good reasons to discriminate apart from our distinction of them in terminology
Indoctrination tends to mean teaching someone to accept beliefs without critical thinking, you're talking about instruction, which is more open to criticism in the first place versus telling someone they must believe something, which isn't what public education would be doing anymore than they're saying you MUST believe in biological evolution in a science class
There's no need to polarize and you can teach a child that: differing opinions doesn't make the wrong one your enemy necessarily or that it must be regarded as such.
The problem becomes how society at large isn't necessarily encouraging critical thinking, even if educational institutions might be doing so. It's a broader issue you're pointing out and trying to make it primarily about public schools coloring worldviews rather than culture at large doing it
