Using monotheism and polytheism as a metaphor here was a poor choice. I really get tired of the myth that the polytheistic cultures of antiquity were much more religiously tolerant than the later monotheistic cultures. This is a myth caused by an appeal to common sense that is not supported by the evidence. Let's take Roman society as an example. They may have had plenty of gods to worship, but there was an official…
The Roman ruling class objected to Christianity because of its politics, not because it wasn't the "state religion". It absorbed many other cults (e.g. Serapis, Isis, Mithras, see the Wikipedia article on "Religion in Ancient Rome"[3]).
Of course, intolerance goes hand in hand with religious fanaticism in any time, but the monotheistic religions are inherently more intolerant because of their rejection of anything other than their singular god. For the Romans, religion wasn't necessarily a fanatical pursuit, but a way to honor their ancestors and part of their politics. They were open to new religious ideas, as long as it didn't mock their way of life, and often adopted foreign cults as their own. In a way, very much alike to many modern believers, who take a little bit of everything in order to create their own, vague interpretation of spirituality.
[0]: http://en.wikipedia.org/wiki/The_Bible_and_slavery
[1]: The divinity of the emperor was a complicated subject that can't really be compared to current religious worship in monotheism. Unlike the uneducated impression I have of Japan, for example, virtually no Romans actually believed that the emperor was a god in the modern sense of the word. It was mostly a honorific title, going back to Alexander the Great who was the first in a line of Greek/Roman rulers to explicitly compare himself with a mythical figure (Heracles in his case, who was not a god). In terms of absolute power over a major part of the human population at the time, however, the Roman emperor (and his Chinese equivalent) were almost like gods.
[2] http://en.wikipedia.org/wiki/List_of_messiah_claimants, http://en.wikipedia.org/wiki/Jewish_Messiah_claimants.