France will get asked by Portugal before they get asked by you. They will only join you if by some miraculous reason, they decline Portugal's call. I don't even think such a hypothetical situation is possible. Crushing dept and/or war exhaustion would be required and would likely cause France to dishonour your call too.
It's possible to block France's ability to join and still have them join. I did it as Aztec before I betrayed them in Sunset Invasion (I was Protestant). This can rarely happen if the AI is willing to join a 2nd war while another one is active, as if you're on the same side as France they *can't* join against you and thus will answer your CTA.
There's one other oddity I observed in that game. If I declared on a Catholic CN with Catholic France DotF, they would join my CTA (ironically the CB was cleansing heresy ^_^). Later on, when declaring on Castile directly, France still had the green check...but joined *neither* side and we kept the alliance. There's some strange stuff going on there with DotF, or maybe it was just cleansing heresy screwing with it, not sure, but I certainly was able to bring France in on a member of its own faith via declaring on a CN that was also of its own faith, which in theory shouldn't work.
