Hmm, yes, I spoke in over haste. It's not the existence of the multi tag that's indicative of a monogamocentric worldview, it's the idea that tag is sufficient to cover all the possibilities it denotes: cheating, serial monogamy, polyamory in the sense of multiple relationship pairs going on at the same time using the same characters, polyamory in the sense of threesomes and moresomes, as well any grey area between those categories.

And fully agreed with you and melusina that cheating fic and serial relationships would be offensive if tagged poly.