I think the book is potentially appropriate for middle school and definitely for high school.
The thing is, books like these provide information on how to do the things they're going to do... safely.
"Gay people exist". No shit.
Now the 16 year old kid who IS gay thinks "Oh man, I would love to do this"... they can either just... wing it and figure out, or maybe have some guidance about how to do it in a relatively safe manner?
Let's just use a different subject to make it make more sense.
I'm... 15, and I want to shoot guns. I obtain a gun. Is it better if I just... screw around and figure it out on my own, or maybe read a book that shows me how to properly do so in a safe manner?
We get worked up because of "THE GAYS!" but really, it's just a book showing how to do "potentially dangerous thing" in a safer manner.