Okay, Let's take your "adversarial relationship" definition and assume it is correct.
If I'm understanding this correctly, the definition of the adversarial relationship the rise of an alliance of two hostile parties bonded only by mutual interests/common goal. When agendas and interests change, the two will have to break up.
By your viewpoint, it fits well to the model symptom of low interests and women cheating and going for bbd. However, your theory I suspect also fits well to friendship as well. By the definition, one can say your best friend is only a fragile bond held together by some glue on the surface. Familial relationships are the exception by genetic imperative.
This generates some flaws to the theory.
1. First is that without the common goal/interest, the base mode is then hostility rather than neutrality. This point is negated if you think the common stranger is regularly hostile to everyone unless they see something to unite or exchange. Currently, I don't think people only act friendly only to people who have immediately value, some do, but many are friendly in general. There's an element of exchange and each individual interests as well, yet it seems a healthy friendship end as harshly when interests fade (and many relationships don't either).
2. As many pointed out, many downfalls of relationships seem to be caused by the fact that many (many throw in American) women are self-centered. She sees men as only a tool and willingly pay, including acting all lovey-dovey and saying "I love you" until the need is over. This seem to be a contradiction to the nature of relationships between a couple is adversarial because the conscious behavior point to a women's character. Seems a good point that many are just being adversarial rather than by nature.
3. To see a reason why most relationship fail and a few succeed, we have to look at the difference between the two. Which is likely impossible to pinpoint due to so many variables include culture, religions, socio-economics factors, but it is possible to look to see if we can rule our possible ones. However, can try to rule out reasons and differences. Looking at the difference, those that do survive, last a long time, and those that end at least amicably does not always end when agendas change. Seems to show character is a large factor and if it nature, than it should apply to all relationships.
4. If we are to assume the core reason most relationships fail is because of its nature and cite everything else as it symptom, then why does some gaining the same "symptom" survive and others fail. If it is the core and the cooperation is just the surface, any bad weather should be enough to wash away any glue that hold the bond, many don't end as bad as saying "I love you" one day and the next day is the new boyfriend. At least by my experience, many times women do feel remorse. It fits well to your experience it seems of the people you been with saying "I love you" one day and then marrying a new man when you don't fit to her desires in a very short time and that does sucks, but there are many examples large enough to say there are breakups that either show a long decline enough that when its finally over, one can see the adjustment have already passed in the wind down or they do enter remorse themselves when things ends as well. It seems you watch girlfriends break up and went quickly to marrying another man with no pause (I'm too young to see that currently it seems), but surely you seen others act with remorse or a long winding end. Could it be more possible that the women just view relationships and men as tools and thus avoid attachment rather than its nature to why so many break up so coldly? Adversarial relations should point that all endings should end without remorse as there is no attachment. To discount break-ups for low attraction in favor of changing agendas means that changing agendas have to be the reason for the majority of break ups. I believe there too many broad reasons to just saw it is the nature of it.
Ok, the above too way too much of my time to type. So I'm going to give a quick alternative.
Aristotle listed three types of friendships:
1. Friendship of Utility
2. Friendship of Pleasure
3. Friendship of Virtue
Now, he only applied it to friendship only and factors relationships differently as it is not equal. However, applying it to relationships.
Well, the friendship of utility is only a connection based on need for something. Now this is adversarial as they are only friends/couple for an agenda or need. Obviously, to mistaken for something more is very damaging when the need is gone. I suspect many relationships these days are just that and this is why it seems so many are adversarial and many end so badly.
Friendship of Pleasure is a friendship that is based a connection based on something internal and enjoyment is on activities together. This is a genuine connection where the person is friends with the person and not just thinking of what the person offers.
Friendship of Virtue is a friendship based on the connection that is not just internal like pleasure but built on a long connection and requires both parties to have strong integrity and good character which is required to view the other self as an extension of themselves rather just a resource and also willing to be open to form such a bond along with several other reasons.
Since of course to cultivate such a bond require such time and rare type of people, may I present it as an alternative to your viewpoint.