cantoresq got it right on the weddings.
“But it is a good example of how all these arguments started, we assume something is assur or muttar, but assumptions are dangerous.”
Correct. And in the case of entering churches or heterodox synagogues, there are indeed exceptions to the general prohibition. In my neighborhood the rabbis endorsed entering the Conservative synagogue for a blood drive where testing for bone marrow matches to save the life of a frum man was being done. It is a *mitzvah* for a recovering alcoholic to go to an Alcoholics Anonymous meeting in a church basement. (Everyone I know who has asked a shilah on that has received the same answer: Attend any AA meeting held in a church UNLESS the AA meeting is being held in the main church sanctuary.) And four Orthodox rabbis attended the funeral mass of the Pope at the Vatican.