Taps forehead significantly.
There’s a group for gay Catholics, “Dignity,” that’s been around since 1969. Like the OP, I’ve never understood why they don’t just get out. is their adherence to Catholicism that important to them, that they can’t be happy with an accepting denomination? Apparently, it is.
Erm… sorta.
There are two kinds of rules. No rule the Church has ever propounded as infallible has ever changed. But there are plenty of other rules regarding, say, the color and order of vestments, and you’re right: they have changed. Tomorrow, the Pope could order that all priests celebrate Mass in Edwardian-era dinner jackets and white bow ties. That would be a perfectly valid exercise of his power as the head of the Church on Earth, but not a statement that previous Masses were invalid.
So that’s one thing – keep in mind the distinction between rules that codify Church practice and discipline, and rules that arise from definitive truths.
Now, the former – the vestments rule, let’s say – can certainly be changed, and can certainly be influenced by conferences of authorities. But even those conferences and those authorities are guided by the Holy Spirit to act in ways that are necessary and appropriate for the times in which we live. So it’s not precisely correct (in my opinion) to simply say, “Hey, if they wanted to, they could change it.” That’s not the case. Each person participating in such conference is, or should be, prayerfully considering how the Holy Spirit is guiding him.
I certainly make room for the possibility of a rouge bishop saying cynically, to himself, “Fuck it – I’ll recommend what I want, in ways that help me, or that I alone believe the Church should go.” But those cases are not by any means the norm, and even that bishop is, though he would deny it, serving the Holy Spirit.
I imagine it’s because they believe that Christ charged Peter with being the rock upon which His church would be built, and they believe that this mandate was transferred to and carried out by each succeeding occupant of the throne of St Peter. In other words, they aren’t simply saying, “Eh, this church seems nice and the people are friendly.” They are (I imagine) saying, “Even though this church isn’t telling me what I want to hear from a church, they are in fact the one true church.”
The idea that they can just choose another denomination suggests that they’re attending church for reasons other than theological conviction.