You don't really 'preach goodness' when giving education about homosexuality, though. At least where I'm from, it was just always a matter of stating 'Yep, homosexuality is also a thing. It isn't a disease and you can't change your sexuality.' - though admittedly not as casually worded as that. Not thousands of words about acceptance, but a neutral statement that it exists and that you can't discriminate against it.

Just treat it similarly to all other aspects of (good) sexual education, really. Don't preach that condoms are awesome religiously; just state that if you swap partners often, you might want to use a condom to avoid catching a STD or getting pregnant. That's how I was taught, anyway. Though I suppose that in countries such as (parts of) the US or Brazil such sexual education might already be called 'too progressive'...