State your complaint, don't harp on it, but then move on to explaining how it damages your appreciation of the book and the narrative, cite examples from the text, finish with a compliment about what you liked.

Find your own voice, look to published literary-reviews for any stylistic questions, stick to the text and interpretations/criticism of the text. As hard as it will be, stay away from the real-world political or social aspects of that fault in the book unless they're easily and directly related to the text. This last one is the hardest one but you undermine your point if you start talking about real-world examples or trends of anti-gay discrimination that have nothing to do with the text because people will go "hey look at the preachy person" and tune you out or make their take-away from the review about you and not your points. Unfair, but true.

Post it here after you write and before you publish it if you want feedback.

Shall do.

Thank you.

Well, maybe you should use some less colorful language.

As for the review, I think it is entirely appropriate to "take her to task". Is there a point behind making him gay? Perhaps it is a necessary component of the story line, but your perception as described in the OP would indicate that it wasn't obvious or perhaps even non-existent. Before you send the review, reread the entire comment replacing "gay" with "black", "Mexican", "Asian", or any other stereotypes you can think of. If your comment passes that test, then it is ready to go. The review won't be taken seriously by the author if you let it drift into "you're a gay-bashing homophobic idiot" type language and it really should serve as an awakening to the use of stereotypes as much as it is a critique of the overall piece of work. If you liked the story otherwise, be sure to say so.

"I faaaaahhhhttt in your general dirrrrection. Your mother was a hamster and your father smelt of elderberries!"

That's just a last resort, mind you.

Though I think my dad did smell of elderberries (or cherries anyway - he loved his pipe.)

And even if an author doesn't consider the other reasons for avoiding such a choice of villain, just attempting to avoid writing a cliche should be enough.

If the character was an entirely different, very well fleshed out character who happened to be gay, then that would be something totally different.

I am not going to read the book, but I guess from your post that the character is Gay Just To Be Creepy. As in he makes inappropriate sexual advances towards other characters, and we are supposed to assume that is because he is "gay".

There are legitimate reasons why a person who is gay in a homophobic society might become anti-social. If the author wants to discuss this connection, good for him or her.

Note: in good speculative fiction there is no "villain". Characters are fleshed out. Some characters do bad things, but calling anyone "evil" is the ultimate act of evil. So, authors who create "evil" characters are doing something naughty, sort of like the Nazis, except with words instead of gas chambers.