Also keep in mind the Catholic Church is not a monolith. Plenty of practicing Catholics disagree with the church on one doctrinal matter of another. Plenty of Catholic women use birth control. Most Catholic couples are probably not virgins when they marry. A Catholic church is not going to physically throw you out the door if they know you’re a trans woman. If you showed up to mass dressed like you were going to a pride parade they might toss you out. But if you respectfully go there and fit in with the vibe of the place, they’ll let you practice. The worst that might happen is a particularly conservative priest might refuse you communion. But most priests would rather have a “flawed” parishioner in the pews than throw them out on the street. The whole core idea of Christianity is supposed to be that we’re all sinners. Most churches would happily let a literal murderer in the door if they confessed their sins and had paid their debt to society.

As far as why anyone would do this? Why not leave the Catholic church for a more affirming congregation? People like churches for all sorts of reasons. Some just like to worship in the high mass style found in Catholic churches. They like the robes, full choir, formal procession, and the otherworldly aspect that Catholic mass often embodies. Some like that the Catholic church places more emphasis on good works than most protestant churches. Some just like the idea of being part of an institution that has been continuously operating for nearly two millennia. For many, especially if they were raised in the church, sticking with it, even if you don’t see eye to eye on everything, is preferable to switching to a liberal protestant church. For many, it’s “my church, right or wrong.” They would rather stay Catholic and push for the church to reform than to leave the church all together.