So now that we’re deep in the weeds - let me reiterate the original question: is there a specific infallible source to point to in an argument with a self-identified Catholic that conclusively demonstrates that the Church has always condemned homosexual sex acts and/or abortion and always will?
The answer is “no.” The issue of what is infallible and not in Catholic teaching is complex and nuanced and the Church declines (probably for that reason, among others) to put out lists of what is “infallible” and what is not. Catholics are all called to follow
ALL of the Church’s teachings, not merely those flagged as “infallible.” It is absolutely clear that the Church condemns both abortion and homosexual acts in the strongest terms. Many Catholics disagree with those teachings (and others), but those Catholics can only admit that they are in dissent on those teachings (or any others)- and should take little solace in the hope that they may one day change.
If your friends are trying to say that it is somehow “OK” for them to disagree with the Church on those teachings (because they might change), my own strategy would to tell them that they are clearly dissenting from Church teaching (right now, which is what matters) and that they must square that with their own conscience and their own confessor. There is no list of supercharged teachings that can convince those dissenting based on conscience to change, merely because the teaching is on that list.
If I can rant for just a line or two - it bugs me that this discussion only comes up with regards to sexual related sins. What I said above is equally true about
ALL the teachings of the Church, including the teachings on social justice, the new teaching on the death penalty, and so on. Catholics may find that their conscience leads them to dissent from time to time, but you can’t just shrug that off and decide it doesn’t count because the teaching relates to social issues.