Well in my first marriage we were monogamous for 11 years before being other. Luckily we were on the same page about thinking that non monogamy would great, but I can certainly see how somebody would have no idea that they'd be interested in more than one person until it happened. I sure didn't.
But yes, trying to manipulate somebody who wants monogamy to embrace poly when they don't want to is kinda shitty. I certainly could see why you'd want to do it though, nobody wants to lose a person they love so it makes sense to scramble. Most people do want to have their cake and eat it too. I just feel bad when the falling in love with somebody else comes before conversations about non monogamy.
I think that it's great as alternatives to monogamy start becoming more known and understood that more people will know there are options and discuss them before getting married to hopefully prevent stuff like this happening as often.
Happiness will never come to those who fail to appreciate what they already have.