I was reminded of this yesterday, when someone rated a lamp with two stars of the possible five, because it was small. The dimensions of the lamp, the shade, and the lamp and shade assembled were in the product description, so how is this person's inattention helpful in informing other potential buyers of its quality?
I've seen the same thing on art prints ("I didn't read the description closely and was disappointed it came rolled up and not framed."), clothing ("This sweater isn't wool!"), and coffee makers ("This didn't grind the beans, which is what I wanted.")
And of course it extends to book reviews.
Don't get me wrong. I'm firmly in the corner of anybody who has an opinion and backs it up, even if I disagree with the opinion and the reasoning that led to it. But I have little patience for the buyer who writes a bad review when a book was exactly what it said it was going to be.
I've seen it in hard-boiled private eye novels ("This book has so much swearing and violence I had to put it down."), horror ("The gore made me sick--like the author must be!") and erotica ("This book is disgusting and decent people don't do these things.")
Excuse me? This is only a small portion of what real people do--your neighbors, the kind people at your church, the clerk who takes your money or sells you the ticket, the couple that owns the coffee shop, the plumber who'll come out in the middle of the night, the receptionist at your dentist's office, the ordinary people whose paths intersect yours on a daily basis.
If you approve only unadventurous sex between married heterosexual couples, then maybe you should be buying erotica only after reading the blurb. There's hot fiction written just for you--and plenty for everybody else.
The erotica writers selling commercially- and self-published books cover the full range of sexual activities actual people do. Your disapproval of their choices, or of who they are, does not belong in a book's rating.
Rate it poorly if it's badly written, if the characters seem flat, if it bored you, if the plot had holes. That book deserves a low rating. But the fact that you do not approve of the activities or characters depicted? Giving such a book a low rating just shows you're a fool.