Well, quite obviously, no, we're not all cut out for lifetime monogamy. I mean, obviously we're not. Just a cursory glance around you bears that out. Breakups, divorces, affairs, the amount of unhappy couples with one half enduring bullying and violent behaviour from a partner who clearly doesn't care very much for them at all on this very forum shows that.
And I say this as someone who's been lucky enough to have had a very stable, loving upbringing, with barely any divorce in mine, nor my DH's family. Anyone who's gone out into the world, however, knows that this is very much not an across-the-board experience.
It is weird that open relationships are considered odder/stranger/weirder/more worthy of looking down on than affairs.
I do think human relationships have changed a lot in the last 100 years, what with divorce now being par for the course, pre-marital sex no big deal, and 'living in sin' pretty much expected these days. I am pretty much of the belief that in another 100 or 200 years - maybe even 500, to be conservative - lifetime monogamy will be considered quite quaint.
Yes, a mutually loving, satisfying monogamous relationship with someone you like, respect, love, lust and care for (and vice versa) is fantastic, but not that many people are lucky enough to a). find that, and b). sustain it long-term. And given how much relationships have evolved in the recent past, I honestly don't think it's going to stop evolving where we are right now.