Ok so no men in women's prisons. But even if they have fully transitioned? And so the flip side is that women that have fully transitioned to have all the male genitalia must be kept in women's prisons? Are we happy with that??
Ironically one of the issues with accepting "gender" transition as sex transition, and so having a policy of putting transitioned people in with the opposite sex, is that the outcome is very different depending on sex - the sex of the people who transition, and the sex of those who have to accept them into their spaces.
If a trans man goes into male spaces there's no increased risk to the men, but there is some risk to the TM (for example risk of being raped). If a trans man is in female spaces there is some increased risk to the other women there, because testosterone may increase aggression and strength. But not as serious as the risk from a male. Phalloplasty doesn't give you a penis that can be used to rape someone or impregnate someone.
If a TW goes into a female space, the risk to the women there increases and the TW is at very little risk.
Same with sports. A TW in women's sports is at an advantage, and even if they don't win, they take places on teams and squads from women. A TM is highly unlikely to have this effect on men's sports (though I still don't support it, as if women should have their own category, so should men IMO - it's not just about the results but the changing rooms, culture and men's right to have an all-male space in something that is supposed to be for men).
There are TW who have fully transitioned and remain aggressive to women, such as Sarah Jane Baker. The criterion for being in a women's prison should be being female.
I think there's a case for third spaces in many of these situations, but many trans people reject them. Even though TW often claim they're unsafe in a male space, they don't want their own safe space. That shows it's not about safety, but validation. Safety should come first.