What happens if you enter an alliance with the Papacy?

So, I have the delightful pleasure to marry off my non-heir son. One of the options is a courtier in the Papacy with less-than-admirable attributes. Still, it sound like a fun thing to do. Has anyone tried this? Is it any different from any other alliance?

I imagine that the Pope would attempt to bugger me into joining any of his little disputes ... and that he'll likely not give much help in return.

(Plot twist! It's actually the legitimized bastard daughter of the Pope himself and some lowborn promiscuous lady.)