What you are describing is a sexual fetish. Sadomasochism or BDSM has long been studied by psychology. Both Freud and Jung have elaborate theories to explain this sexual fetish. Most theorists since then also have had their theories of this sexual fetish. Generally, we need to assess the effect of the fetish on the life of the individual. In other words, is the sexual fetish causing problems in the life of the individual? In your case, you are experiencing guilt and this is a problem.
Many people include this particular sexual fetish in their normal sexual lifestyle and it has no negative effect upon the rest of their lives. Your fiancé may share your interest but you will not know this if you do not talk with her about it. From your letter, it appears to me that you may be ashamed of your fantasies and thus would not want to reveal them to your fiancé. Many people have sadomasochistic fantasies and many of them incorporate these fantasies into their sex life.
If your fiancé had these fantasies and you did not, would you think less of her? If you think that these fantasies are bad and unacceptable, then it creates a dilemma because you are enjoying viewing them on the Internet.
A therapist could help you to stop viewing these materials or they could help to remove the guilt that you feel after viewing the materials. I can tell you that your sexual interest in this fetish is fairly commonly found in the world’s population. I cannot tell you that you should accept these feelings as normal, if you find them to be unacceptable. If you feel that your fantasies are inappropriate, then through your own efforts or with the help of a therapist, you should stop viewing this fetish on the Internet.
I would highly recommend that you discuss this with a therapist. I wish you the best of luck.