I respond to the deliberate infliction of pain very differently depending on the setting. A fantasy can be fairly extreme but I know that it's only a fantasy. If it's a description of a real-life spanking, or a movie or sound clip of one, my gut tells me whether I will perceive it as OK or not OK.
Green light: if the bottom consents and the activity is safe I can find pain very erotic.
Red light: if consent is absent or the activity is unsafe it gives me the creeps; it squicks me, in BDSM-talk.
And if I'm not sure what the circumstances are, I hold my erotic response in abeyance and look for clues as to the color of the light. A friend once sent me a graphic description of a prolonged and severe paddling. I was sure it was safe; but as I read, I couldn't tell whether it was consensual. I remember thinking to myself, "this might be very sexy . . ." I had to ask her for more information before I knew if it turned me on or not.
Trish once told me that she first found this site (www.doctsai.com) via a link to the sound file of Sandy's paddling (Sandy's Paddling: Six Minutes of Agony). She opened the page and listened with "deep arousal and deeper horror." As she said, "I had no context for that experience, for her pleas, for the nature of your relationship with her."
She left my site for some time. When she came back, she read Teen Guide to Spanking, About Me, and much of the rest of this site. She learned more about me and the spankings I promote or inflict. Now she understands, and recently she wrote me, "I particularly like sound clips, and have revisited the Sandy's sound files to see how I feel about them now, in context. I had a completely different reaction the second time--I enjoyed them, and understood much better where Sandy was and where you were in that scenario." (2008 update: Trish has now visited, more than once, and knows first hand what it's like to be spanked very hard indeed.)
One of my major goals in this site is to help people understand that it is possible to experience real spankings while remaining entirely within the realm of the safe and consensual. These real, but consensual, spankings are, for many of us, far more erotic than a spanking that might be more severe but in which consent is blurred or absent.
I have additional remarks about pain in the subsections of this page (see links below) and also scattered throughout the site. One good example, if you'd like to read more of my thoughts on this topic, is at 3 The Taboo Against Pain in the discussion of Cat's punishment (By God).