The story's premise - a virgin hero who has written a book on male chastity brought down to earth by a cynical and weary courtesan who wants to leave the "profession", but needs one last trick before she can do it - is interesting, and the hero is brilliantly characterized. He's not at all stuffy or full of himself - he's actually completely honest with himself and he's used his training in philosophy to think through the dubious morality of the Victorian double standard for men and women, and what it really means, especially for women. This is the aspect of the book I liked best, and it's very, very good.
The hero has the misfortune, because of his book, of being a huge celebrity, is mobbed everywhere by his "admirers" and has no chance of having an ordinary relationship with a woman. I liked this aspect of the book quite a bit less, although it's fairly amusing to read the author's take on how a celebrity would be treated in Victorian times (chased by reporters and admirers, no privacy, etc etc).
The heroine wants to ruin the hero by seducing him and is getting paid to do this. In the end she doesn't do it because she falls in love with him. The point in the book (about 2/3 of the way through) where the hero has fallen for her, and she tells him the truth about herself, causing him to dump her, is the high point of the book for me. After that the book just tries to tie up the various threads of the story and brings in the hero's family, which didn't work nearly as well for me. Much of it seemed contrived, although there were some good scenes, especially the hero's "surrender" to the heroine. I didn't feel emotionally connected with the story or the characters in the last part of the book, unfortunately.
This book could be compared to Mary Balogh's "No Man's Mistress", which also has a virgin hero and an ex-courtesan as the heroine. Mary Balogh's book is one of my all-time favourite romances and grabbed me emotionally right from the first page. The ending is truly sigh-worthy. And the seduction of the hero - oh my - you'll have to turn up the A/C (or go outside for a brisk walk if the weather is cold)! Balogh doesn't get into a discussion of the double standard for men and women - she accepts that it was just there at that time in history - but she does insist that the heroine is vindicated and gets the justice that is due to her.
I should add that I read "Unclaimed" on its own merits and was not comparing it with "No Man's Mistress" while I was reading it - the comparison came several days afterward when I realized the similarity between the 2 books.