I feel like advice that was given to me about writing characters of the opposite gender also applies here. Just write it as if it were a regular kiss with individuals of separate genders. Of course you may need to make exceptions for anatomy, but I think what would be more important in this is the build up to the scene, rather than the scene itself. Regardless of character genders in any book, if it doesn't seem like the natural progression of the story, then it may just come across more jarring and you would loose suspension of disbelief for the reader to go, "what?".
If the readers are already turned off by the existing chemistry, likelihood is they aren't going to reach the end of the book anyway. With that in mind, I personally feel like YA novels that drop big relationship surprises at the end really turned me off from a series. If there was a buildup that I wanted something to happen and it finally happens in the last few pages, it feels satisfying. However, whenever I have read a series that tried to drop it as some sort of twist hook for the next book when it made little sense for the story, I usually just dropped the series all together. I'm talking to you James Patterson.