The fact that you are aware about this makes it workable. If the nature of your thoughts and imagination is intrusive i.e. if this gets stuck in your mind causing distress and reoccurs frequently then it might be OCD.
Also, if this is disturbing your day to day functioning then it is a concern that needs to be addressed.
Next Steps
You should visit a nearest Psychiatrist or Clinical Psychologist for further evaluation.
Health Tips
You need to be more aware of your thoughts and your sexual fantasies as well. What you indulge into might encourage the pattern to persist and become more difficult to get rid off.
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