Right now, changing other people is a very complicated skill for the MC. Changing himself is easy because it's his own body, fully under the influence of his superhuman nature. But for others, the natural tendencies of the body are hard to manage without fucking up everything.
Giving his own appearance to a willing person was one thing. Changing a willing person (the inmate in question) into the opposite gender was another, likely successful because they actually deeply wanted it (I'm not gonna go into details but basically in the ballpark of that person being trans so the subconscious and body didn't reject the change).
Changing someone not willing, or not fully embracing it at least, is likely to cause complications that could lead to horrible outcomes.
And I doubt the MC is willing to take the risk of maiming friends just for a fuck-session. Plus there aren't any guys amonghst his friends who'd want to do that, pretty sure.
This starts right and then goes wrong.
MC experimented on hundreds of inmates before finally mastering the technique. allowing him to change other people.
this is not "because they deeply want it". it is because he now has the power to bodysculpt others
It is still very hard to do. but it is a skill he developed. and he developed it on fully unwilling inmates.
He also explicitly demonstrates it on unwilling inmates. if you spare alpha breaker, you modify him into a dickless ken-doll against his will. and tell him if he behaves and serves you well enough you will restore him
MC is also explicitly willing to use it now that he has mastered it. Even on the unaware
He was going to use it on mia (but ella stops him and does it herself). And he may optionally use it on the whiny friend with the tiny dick (to give him a bitch breaker). Both were unaware of what he was doing and thus not actively consenting the way the M2F prisoner was.
it is also not really "maiming" anymore since he can just undo any change he makes to them. As per the whole de-dicking of alpha breaker and statement to one day restore him.