I will confess to having only skimmed a bit of this thread and apologize for jumping into the discussion, but here’s my answer to the original question, “Where does Catholic act-based morality come from?”
A few basic principles…
- Creation is good.
- Created things have in-built (“natural”) purposes which obtain for those things their proper good (“happiness”).
- We can know these purposes and their order.
- We can choose to respect them or not, which has “moral” consequences, and of course is done through acts (which may be interior or exterior).
I’ll stake a bold claim (that I am not prepared to defend, so please be nice)… Nearly all opposition to Catholic morality (throughout the history of moral philosophy and theology since the 1st century) contradicts in one way or another one of the above 4 principles, or is a fight over the particulars of #3.
There is much confusion these days over what “moral object” is. Let me use an example to illustrate.
I believe it was the very same Paul VI that both wrote Humanae Vitae and urged a group of religious sisters living in a dangerous area to take contraceptive pills to prevent pregnancy by rape. Can you see the difference between this act and “normal contraception”? This example strikes the very heart of the matter…