Someone who uses the term 'true submissive' generally believes that there is only one way to be submissive: their way. Unfortunately, this type of thinking can take away from the BDSM experience. There is no such thing as a 'true submissive' because there are varying types of submission and submissives.
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While there is nothing inherently wrong with finding some of the 'true submissive' ideas something that you may enjoy, there is no one right way to be submissive. If you identify as a submissive and someone wants to push you to do something you don't want to do by saying, "A true submissive would do it," then you should not play with that person because it is likely they will not respect your individual wants and needs. BDSM, ultimately, is about meeting the needs of the partners involved. Since all of our wants and needs are different, there is no such thing as 'true submissive.' It is an inappropriate ideal.