If monogamy means 'no feelings for anyone else', yes I think that's unrealistic. I think people in happy monogamous relationships can have intense crushes on other people and enjoy that feeling while not acting on them. Emotions are weird. There are people I had a crush on a few years ago that now I feel nothing for or am happy not seeing.
With lifelong, as I said on the thread about whether affairs are more common, I think that an idea that some have of teenagers still in school being in monogamous relationships that will last a lifetime does more harm than good. There are the outliers, but I think there are benefits for young people having non-exclusive relationships for a significant amount of time before making an active choice to go monogamous.
I think in general monogamy being seen as an active choice that has benefits, risks (like not having that first rush again), and responsibilities to be agreed upon in a more detailed way than often portrayed rather than the default romantic option because a pair of people like each other or have been on X amount of dates would mean a better chance for those who choose it.
Beyond that, I think it's a mix of individual personality and environment. For some, maybe it's not realistic at all, society has somewhat moved to a serial monogamy assumption and polyamorous relationships are becoming somewhat more accepted which may encourage some, but I know others in open or polyamorous relationships who identify that way and still live for many happy years as monogamous because of no real desire to deal with dating & the thrill of the new doesn't appeal either but are just 'open to the possibility' often after a while of having been in non-exclusive relationships or really only want affection friendships (meaning holding hands, cuddling on the couch rather than FWB-sex 'affection') with other people but as there is largely an idea that those types of affections are only for romantic partners so they find it easier to find others who also enjoy that type of platonic relationship through polyamorous communities.
People are complicated, our relationships more so, I'm not sure we can pin much down as entirely unrealistic.