There's a certain mode of Roman Catholic argumentation made popular by Newman's Essay on the Development of Christian Doctrine which seems to me ass-backwards. Schematically it looks like this:

1. We know doctrine X.
   
2. We cannot know doctrine X unless the Roman Catholic Church is infallible.
   
3. Therefore the RCC is infallible.
   

Usual tokens of X include Trinitarianism, Chalcedonian Christology, the canon of scripture, and the normativeness of monogamy. So Newman tries to make the Bible and the Fathers ambiguous on the divinity of Christ, hoping his (largely Anglican) readers will get worried and hide under the Pope's wings as the only refuge from Arianism. It's interesting, by the way, that at the time of its publication the Essay was very poorly received by conservative Roman Catholics like Orestes Brownson, because the usual view up to that point was that Catholic doctrine had been handed down in toto since 33 A.D. through the apostolic succession of bishops.

In any case, the Newmanian argument is clearly circular: if (2) is true then how could we know (1) unless we already accepted the conclusion?

Still, I admit I haven't seen a good biblical argument against polygamy, so Protestants may be relying more than they know on Catholic tradition.