I didn't say you think that sex is filthy. I just thought maybe people who enjoy being tied up were told that enjoying sex is wrong/selfish/whatever, and therefore they get off on having it "forced" on them. But that doesn't explain why I would like to try bondage, so I don't know if that's an accurate theory.
I don't think it's necessarily a psychological problem in most cases. I suspect people are just wired very differently when it comes to sex. For instance, many people like dirty talking, which is a form of non-physical abuse. I can't personally imagine that being a turn on.