Just because you don't need convincing, doesn't mean that the purpose of the movie isn't to convince. There's a difference between art designed to make you think about an issue and propaganda designed to sway public opinion. Does the line between the two get blurry? Damn right! I don't think it was blurry in this case. The other view was represented by an effeminate, foul mouthed, platitude spouting ineffective priest.
It is funny I thought the preists argument was effective. And I think the cursing by the preist (I don't think it was the only cursing in the movie but it was pretty free of it) was to show the relationship he had with Eastwood. Even the most saintly of people are going to break down at some point when they know someone has been arguing for arguings sake for 23 years, but when Eastwood is faced with a problem the preist is there for him like a friend would be.
I don't feel it is propaganda is because it shows that it is not a consequence free decision. The preist tells Clint what will happen, and that it seems is what did. After he does kill her he can't remain in LA and be reminded of her. He has to leave there, and get away from the pain. Much in the same way that it seems he left his real daughter at some point.