To Matt's and Paul's responses, that brings to the fore the question of behavior and disease, and we live in a society that has been focused on venereal disease long before the advent of AIDS in the West.
Couples, and dating implies couples to some degree, have always dealt with the gamut of pairings, from monogamy to open relationships. And, there have always been people who presented as monogamous but didn't live that way. Sometimes those were men who had long-running monogamous affairs on the side, and others with more of a herd sexual pattern. Of course, either could be the vector of infection to his partner at home. That has always been true, but it changed when AIDS became a possible "gift" to the partner. To be sure, other VD had been fatal in the past, but not as certainly as AIDS was before the drugs.
On an asexual line of thinking, a wife today could be taking her children to the park, to the store, to the daycare, and by natural course exposing them to mutated flus, the incurable version of TB, or worse, but society doesn't view her behaviors as being avoidable risks that expose her partner to contagion, possible lethal. The risk just seems low and "normal."
Would we view the promiscuous sex behaviors as irrelevant if we were living in pre-AIDS society? I really don't think so, even though there was an era of more active exploration between the 60's and the 80's in much of the West. Society, including religion, is responsible for the norms and mores of pair bonding, but not exclusively. But, at a deeper level, biology and evolution have promoted pair bonding in humans. A species with offspring who take fourteen or more years to come to develop secondary sexual characteristics requires more than one adult to be a nurturerer. Society today illustrates the point even better with the higher single parenthood rate. Sometimes those divorced or unmarried models have stepped up to try to compensate, but more often, they have simply failed the children.
It is not exceptional nor even Puritanical that children grow to adulthood and still retain the instinct for exclusive pair bonding. It isn't universal, nor is it necessarily accompanied by a morale code, but it remains part of the biological imperative, not matter how many studies only consider the sex drive.
Perhaps homosexual preferences DO vary significantly from the heterosexual population due to the much higher lack of bonding that has a goal of procreating offspring. It will be interesting to see data over the coming decades to see if that is true and if there will come changes with the increase in gay-marriage-tolerant societies.
And I think EastofEden's comments about monogamous expectations being tantamount to owning one's partner are challenging for most ears. Theoretically, one falls in love because one has needs. One needs to love and to be loved. EastofEden, as other posters, has reminded us again that needing sex and needing love are different. Granted many people do not separate the two. As I ponder his argument, I keep returning to the question of whether his experience is typical and whether it matters to a potential partner. Assuming testing is trusted and the inbound partner has allayed the concern about VDs, the question comes down to expectations of the bonding, which again returns to the OPs question and to Derek's response. It matters for whom it matters, but the why is more complicated than merely the need to "own" one's partner. For most, the partnering, if long-term, carries with it an expectation of the pairing being more important and therefore being reliable and enduring. Few seem to be able to carry off the polyamorous model with that stability.
Society is evolving to make room for more variations, but it remains to be proven that they will largely supplant the monogamous models that societies have maintained for many centuries and longer.
