"He bit his lover's lip hard enough to draw blood."
And others to that effect.
Is that supposed to be sexy? Creative? Artistic?
It's none of those. If anything, it's clichéd (therefore boring), unrealistic (therefore pointless in most kissing and sex scenes), and just plain silly. Especially when it's followed up by something like, "It stung a bit at first, but as his lover licked at the wound the pain turned into pleasure."
What the hell are they---puppies?
I think it's safe to say that most people don't get off on feeling (physical) pain. I think it's safe to say that most fictional characters are included.
So why is this so common? I will tell you. Because a few people thought it was neat when Someone Unknown first used the phrase at the Dawn of Fandom; they copied it, and then people started copying them, and so fourth. Now it's commonplace. A throwaway line. A space-waster. Neither imaginative nor useful. I cringe if I come across any variant.
I also dislike it when applied to one person. For example, if Harry is being derided by Snape on a particularly bad day, he may bite his own lip in annoyance or frustration. Many times, an author will write that he bites to the point of blood trickling down his lip. The common reaction is for Harry to lick the blood away, without acknowledging his pain. Tell me, how is this so? Don't you react when you stub your toe, prick your finger, or have a run-in with the family cat's little claws? Not to mention pressing your blunt tooth through the sensitive flesh of your lip!
Not to say pain is out of the question, you crazy kids. But surely you can think of some other painful, sexual act to amuse yourself with? What about fisting? There should be more fisting in fandom. :)