What A great OP! Personally I believe sperm competition theory can explain much of this "lifestyle"
Look at studies done in our close cousins, the apes. In these studies A caged Male ape was introduced to A receptive female. Of course the deed is done and after the Male ape orgasms he loses interest in her, her womb is seeded and his body is flooding with tryptophan, he languishes, relaxed and sleepy.The same as A human Male after an intense orgasm. He's performed his biological necessity and it's all up to fate if she concieves.
Introduce another female and the Male ape will "rise" to the occasion however. He lost interest in the first female as she is already filled with his sperm, however the chance to pass on his genes triggers his biological impulse and he will then mate with the second female. Introduce consecutive females and the Ape will literally mate until he dies from sexual exhaustion. The need to pass on his genes is such A biological imperative!
Put A monkey wrench in the experiment (pun intended) and introduce another Male Ape into the scenario. Make the caged apes watch whilst another ape breeds the female and then in turn put her into the cage with another Male. The Male ape will breed with her of course. Fucking her as hard as he can. Trying to displace the sperm of the other Male.
A tangent: in cuttlefish the Male, having claimed A female from another Male by combat, will literally place his jet into her and exhaling, forsing the other males sperm out of her, before commencing the mating process! ?
In some Ape species sexual promiscuity is the only defense A female has against the larger, more aggressive males. She mates with ALL the Male apes in her tribe. That way the males don't know if her offspring is his own. In chimpanzees A Male chimp will ******* on sight, instinctively, offspring of any female he has never seen before, he knows it isn't his and won't stand for it growing up to maybe challenge his place in the sexual hierarchy, or compete with him for resources.
Now consider studies performed on humans. One discovery is that A human Male will orgasm harder whilst having sex with A woman who has been with another man. Not only will his orgasm be more intense he will also ejaculate more sperm as well, all A biological response. If you think about it, why is gangbang porn so popular amongst straight males? Why the obsession on this site with "creampies" and "cleanup" among many here?
Human beings are A predominantly monogamous species, with promiscuous tendencies, as opposed to some species who are primarily promiscuous. Part of the reason for this is the long gestation times for our young, along with the extended time it takes to raise our young, 18 years for the most part. It takes that amount of time to properly train our offspring to make their own way in the world.
There's an inherent dichotomy in human, straight males however. We NEED A connection to A human female. We need love! Take for example A rich, successful Male. He could easily keep multiple females in apartments around for his fun. However even rich bankers and movie directors get married. Not saying they don't have A side piece every now and then, but they commit to A single, human female.
Now imagine your average guy. He does not have the resources to "keep" multiple females and provide for them. His only option is to, as the great Chris Rock has explained, "recycle the pussy" for some men the reason sharing their women with other men is the excitement, all biological, of having his sperm compete with other mens. A typical married couple has sex thousands of times. However let your wife be taken by another man and the excitement level he has for her is reset. He's fucked her 3 times A week for decades, that's gotten old. Having her after another Male has ejaculated in her enhances his lust, his orgasms, his ejaculation!
As to the ir aspect of this I believe it is fringe. Keep in mind that there's under 200k people on this worldwide site and the "cuckold fetish" is much larger than that. In my case it probably stems from the fact that I was abused by A black Male when I was young. It resulted in A kind of fixation, maybe trying to take back the power I had robbed from me when I was A kid.
In others I believe A masochistic streak, or even A racist one plays A part. We all probably got here, in this particular niche, in our own way.
So to wrap up the primary drive for this kink or "lifestyle" is, in my eyes primarily biological, with A social aspect as well.