It's not in the curriculum, but in any kindergarten classroom you will have a couple of kids whose parents taught them the basics of reproduction, on at least an academic level. If the topic of same sex attraction comes up (even just the boys can like boys and girls can like girls part), they naturally are curious and ask about the mechanics.
Too many teachers think they should answer any and every question a child asks. So instead of using proper judgement and saying "that's not age appropriate" or "we don't need to go into that" they answer fully and honestly.
So now we have a class, who half didn't even know about reproduction, who now know some of the mechanics of gay sex. So yes, it happens.
Some teachers can't be trusted to use judgement. I had a few of these myself, though not in elementary, in middle school. They weren't malicious, just too honest. But that was over 20 years ago. Society is even less uptight now.
I don't think this bill is the answer, but I understand parents concerns and how this happened.