Ed Peters (continue to pray for the complete recovery of his son Thom) has an interesting post at his place. You can check the full text and the whole context over there, but let’s focus on the guts of the entry:
Considering her age (+2,000 years), her membership (+1,000,000,000), and her range of concerns (eternal salvation and human civilization), the Catholic Church has a remarkably short list of non-negotiable assertions. Some of these non-negotiable assertions deal with dogma (e.g., Jesus is divine and human, or, there are exactly seven sacraments) and some of these non-negotiable assertions deal with doctrines (e.g., the Church has no power to ordain women to priesthood, or Thomas More is a saint) but in both cases, the assertion being made is, Catholics hold, being made with infallible certainty.
Now, among the assertions made by the Church with infallible certainty, I have argued, is this one: God made marriage to exist between one man and one woman. Catholics could debate, say, whether this assertion is a dogma to be believed or a doctrine to be held, or whether the assertion is knowable by reason alone or requires the gift of faith. Catholics could even debate whether civil unions of one sort or another between two persons of the same sex are good for society or bad. But Catholics cannot, I suggest, argue whether true marriage exists only between one man and one woman. To debate whether marriage can exist between two persons of the same sex is to imply that some Catholic non-negotiables can be negotiated by Catholics.