I wrote my paper on "steam" ratings after attempting to help a patron who asked for "sexy books." I found two different websites that accepted reader-submitted descriptions and ratings for both quality and sexual content. Allaboutromance.com and romance.io both have five point sensuality/steam scales that are comparable, beginning with kisses/innocent and ending with burning/explicit-and-plentiful. The websites agreed with my assessment of Colleen Hoover (however only one site had a listing for the title It Ends with Us) (a 3 rating) and of TJ Klune's The House in the Cerulean Sea (a 1 rating). Helpful!
Also, limited. With fewer contributors, fewer titles are reviewed leaving a mainstream book like Hoover's out. The other important limitation of these sites is that with fewer contributors, the ratings can skew more easily. While both sites provide descriptions of each point on their five-point scale in the interest of objectivity, contributors may disagree. On a site like Goodreads.com, there are hundreds of thousands of reviews which makes for a more reliable score than either of the romance sites, which only receive a couple dozen reviews at most. A final limitation is that the sites focus on romances, which may leave out a lot of other books that contain (or don't) steamy sex scenes. I'm reminded of the blog assignment where a patron asked for a "clean" mystery. Neither site had reviews for mystery author CJ Box, for example.
Finally, what does the American Library Association have to say about rating scales? The ALA Bill of Rights maintains that readers are to be free to choose whatever they want to read. Yes, of course. And that rating scales can be--but are not required to be--included in records with the source of the rating score identified and an explanation that no rating scales are endorsed by the ALA. My conclusion was to keep steaminess stickers off of book spines, but to keep both websites in mind when advising readers who specifically ask for a certain type of book.
On a titillating note, check out some of the titles on romance.io's site--they really go down the rabbit hole of alternative romances, including a whole fictional universe called the omniverse that has its own rules about alpha and beta personalities, and books that are so over-the-top, they made me blush more than Fifty Shades of Grey.