On the surface this is not a complex concept. Forced Feminization falls into the category of a Dominance/Submission fantasy. In this storyline there is a dominant person (either male or female) who 'forces' a male into assuming a submissive and overtly feminine role. The submissive male is often portrayed as unwilling and reluctant.
What makes this fantasy difficult to understand is that the submissive male (while pretending to be reluctant) actually WANTS this treatment. Their protests and demands for the treatment to stop are often surface deep, while their arousal and deepest fantasies crave the fantasy to continue with even more intensity.
As you can no doubt understand, having feminine feelings and desires can lead some men into having a persistent shame and/or embarrassment about their fantasies. Even in their own head, the idea of actively seeking femininity makes them upset and anxious.
But what happens when their fantasy is 'forced' upon them?
By allowing someone else to play the 'villain' and to MAKE the fantasy happen, the guilt that the submissive male feels can be relieved and they can enjoy the concept of acting and feeling feminine.
Don't. Stop. Please.
There is a problem that tends to develop in 'forced' feminization role-play (especially in a public forum like this one). To an outside observer, the submissive male seems to be unwilling... begging for their dominant partner to stop and appears to hate the way they are being treated.
The REAL(TM) relationship between dominant and submissive is exactly the opposite. What is cruel and offensive to the observer is often quite pleasurable and the actual desire for the submissive. Oftentimes the behavior is specifically requested by the submissive and merely 'acted out' by the dominant as a way to fulfill their partner's fantasy.
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