Simple answer: I would never do mono.
Monogamy sounds like an artificial, arbitrary limitation that I don't agree with and do not understand. I prefer both my own autonomy and independence, as well as that of my partner(s), free from such limitations.
Plus, more practically: If a partner desires genital sex, a mono ship with me would mean involuntary celibacy on them, as there's no way they'll be getting that sex from me; and imposing in-cel on someone sounds a cruel and loveless thing to do. Thus, poly and/or open agreements are the key to making me "partner-compatible" with a whole lot more of people in the first place, whose sexual desires would simply take us off each other's list of potential mates otherwise.