The bigger question is why every game assumes that the MC won't wish to become a sissy of their own free will. Sans any coercion. I think I can answer that. It's because in any good story there needs to an antagonist to pit the protagonist against. There needs to be some sort of conflict.
Yeah, I don't like involuntary feminization as well. Not to meantion that most games implement it in a very silly way, that both doesn't make any sense and completely criminal. I think i played only 1 game where actual criminals abduct MC and feminize him with coercion, and i didn't like it one bit, was too brutal and repulsive. Most devs resort to some overused tropes like blackmail, or higly dominant GF that MC can't leave simply because there isn't any other content in a game :/ Result is usually some very strange and nonsensical hybrid between voluntary and involuntary, where protagonist kinda doesn't resist too much, but on other hand is feminized in very degrading and certifiable way.
I think everyone writes like that for 2 reasons:
- they don't know any better, because almost every game haп same cheap plot gimmicks so they naturally adopt them as well, continuing vicious circle.
- it easier to write, because you can completely disregard questions of MC motivation and thoughts on the matter.
To write voluntary game, you'll need good writing skills, and lets admit, usually writing isn't stellar in sissy games. Its kinda amuzing that lacking writing skills in actual text game doesn't prevent them from getting recognition. Here goes my imo, don't throw stones at me, pls. In the games you mentioned so far, only Secretary can boast with good writing. Perverded Education is considered landmark for some reason, but it got some decend writing only when they hired writer that worked on cheerleader routes, all other writing is below average. The Sissy Girlfriend Experiment has good UI and tons of content, but actual sex scenes and descriptions are so insipid that reading them is a chore. The Making of a Slut is complete nadir of this genre, i can't think of any point that game does good. Horrible UI, inept writing, and to top it off with lack of vision, like dev doesn't know what to do with his game.
If you're looking for voluntary game well done, i can't recommend you enough Down and Out with Paris and Marion. Granted MC isn't very willing from the start, but is persuaded with teasing and guiding, instead of coercing and blackmail. Almost every fetish is avoidable and your route is result of your own actions.
