I'm not really sure what the distinction between Young Adult 21+ and Adult 21+ fiction is, other than the age of the characters involved, but I will admit to being kind of excited to see LGBT erotic fiction deliberately targeted at a young, maturing audience.
The Teabagger Inn by Ashlee Rivers is my first experience with that Young Adult 21+ category, and I'm pleased to say I enjoyed it. The story itself is a little rushed, glossing over significant events as it jumps between erotic scenes, and it definitely leans heavily towards fetish/fantasy side of the genre, but it does have a freshness to it that's appealing.
Basically, what you have here is the story of a young crossdresser who only wants to find a hot, well-hung stranger to introduce him to anal sex, and who ends up being blackmailed, threatened, and 'forced' into the life of a high-prices transsexual prostitute. She adapts to her situation with ease, quickly coming to enjoy the expression of her femininity, and ultimately comes to regain control over her fate through some clever dealings of her own.
Like I said, it's a bit rushed, especially with the two big twists near the end, but it's a fun bit of erotic adventure.