Interesting. Life long commitments are still the norm in a significant number of cultures, societies - from the wild plains of Africa to riverside dwellers in India. I doubt there is a 'for the vast majority' answer.
Maybe not down in South London, where there are issues of a proliferation of young black single mothers raising their children on the own, and we have to have the government force dads to pay their due; but go to a village in Bangladesh, where if a person's partner dies, they tend not to remarry. And if you claim the difference is due to society, or culture, then one must come up with an EP theory to explain the differences. And if one does, you have another problem, because you have just explained completely contradictory scenarios, which means, well, almost anything goes if it sounds reasonable enough. If it sort of fits.
The society I currently live in is [London, England] is one of the most highly sexualised societies of all time. Even trying to figure out what is 'normal' is difficult enough. Things seem to be changing by the decade. How are we to decipher the effect of the environment, such as the ease of access to porn, and the plethora of elite sexual 'role models' being espoused in magazines. Super thin. Super curvy. etc etc etc. I think all of that has a part to play. As does the existence of choice. This idea that 'we're not meant to be a certain way' or that we are 'meant to be a certain way' is based on evidence that doesn't look at the picture holistically. And to my mind flimsy evidence. And certainly not enough to decide what, if anything, the human condition is.