Well, this is certainly an oddity. Most reviews would lead you to believe that this played out something like a gay version of a Hallmark channel film you might see on TV one afternoon. And, indeed it does for the most part. If any novel could be compared to one of those TV movies that is pleasant enough but pulls at the heartstrings a little too often, then this would probably be it. As others have said, it's sweet and charming. But I wonder how many reviews were written before they got to the final quarter of the book.
In an almost schizophrenic about-turn, the final section of the book moves away from charming romance that wouldn't even offend Mary Whitehouse to throwing four-letter words at us and giving us rather detailed sex scenes in lieu of a proper conclusion to the story. It's a very odd state of affairs, and rather a sad one too. Part of the appeal of the book's storyline and writing up until that point is that it was a gay-themed novel(la) that didn't rely on copious amounts of nudity and sex scenes for once.
It does appear that very few writers and film directors think that gay men are willing to watch or read any gay-themed work unless it contains moments that are, no doubt, intended to arouse the audience. With The Christmas Throwaway, I would argue that these elements come so far out of the blue that they are almost laughable - or that someone as a practical joke swapped the original final act for a sexy one without the author noticing. It's a shame, because this is sweet and charming up until that point, but the final chapters are silly, supposedly sexy, padding and nothing more.