They make you depressed when you finish them, if you finish them. Then there are the good books. Sure, you love the characters, the plot is deliciously tangled, the style is lean and mean or lush and rich or something else altogether. But they're not great. They're just so great that you don't care.
Sex is sort of like that.
Well, I'm not about to discuss my lover's inadequacies here, or even my own. But I will tell you that writing about sex does have somewhat of the same effect on having sex that writing books generally has on reading them. Specifically, bad sex is still bad sex. You think 'Eh. It's still a pleasure and a joy and a comfort.