This is certainly not meant to enable any fantasy expectations of the real world or to give you something to jack off about.. It is just based on what I have seen and experienced...being single, involved, single again, involved, etc over a number of years:
"Girls?" Maybe 20 % already like it or would be open to it...definitely. I'm gonna go ahead and say ages 15-20 for girls. I have a hard time with the "girl" term extending beyond that age range. There are many sadistic younger girls out there and that trend is growing. They like destroying guys in one form or another (for fun-at least so I tell myself). This can range from - one I knew who wanted ever so much to genitally ******* me. I endured some punishment but when the word cutting was uttered, I found I could no longer partake. - to a less extreme (rather mundane to me now) example of one I knew that seemed to relish in placing sexual restrictions on me and taking every opportunity to mention how a different guy does not have the same restrictions and why he doesn't. I accepted that for a while too. That is a large range of motivation and an individual range of experience, but an indicator of a larger truth. Whether bossing them up or crushing a dude's spirit, the girls are loving it more by the day. Perhaps I deal more with the girls in the venn diagram where modern "millennial" feminism and modern sexual kink-desensitization converge. Basically everyone is a fringe element in something and maybe I play with the fringe more often than some, but that doesn't make my estimate % any less legit. 20 is clearly not majority, so don't start assuming every girl you meet is down for all the nasty ******* you have on your mind.
I probably should clarify that I view cuckolding differently than many here...I don't think a male can actively choose cuckoldry. That is not up to him. In order for it to be true cuckoldry, it must originate from a female and then be enforced or schemed into practice. Something the cuckold believes he must accept for various reasons. Maybe he ends up appreciating it and enjoying it and that is lucky for him, but it is not guaranteed to be the goal. It must be her singular kink or part of her kink collection. Feminine-driven cuckoldry contains elements of sadism, power play, domination, exhibitionism, and yes occasionally the thrill of fucking alpha men guilt-free while being in at least a somewhat stable relationship with another dude. Yes, many more girls are involved in all this or desire it than you might realize. It has definitely been a surprise to me...for a long time a very unpleasant one.
I want to point out none of this necessarily applies to serious "traditional" relationships though. Those numbers would obviously be less. But the OP is implying open relationships, which start and are rather dependent upon one or both partners being ready, willing, and able to...play well with others. So the realm of "relationship" I am applying numbers to refers to the dating "hunting" stages through to open marriages and polyamory.
Women? Maybe 15%
There is a certain thrill of wielding sexual power. It seems the older and more experienced a woman gets, the more likely she is to indulge in embracing this particular thrill when there are opportunities. Older and more experienced often brings more open mindedness, even if they did not grow up ever expecting to be involved in some of the things they end up involved in.
Also the concept of being watched fucking another man and being adored on that physical and sexual level is a rather unsurprising bit of satisfaction to many women who begin to age and perhaps are beginning to lose a bit of the positive self-image they once had. More power to them. Some actually regain a lot more confidence and begin a healthy cycle of inner strength due to the lifestyle.
I am certainly not like many on here who seem to have a fetish for a woman basically just cheating in a "controlled" manner. A lot of addicts and masochists here. Many males here also seem to be rather close to having a homosexual desire for black men. My perspective is coming from a very different place I guess. Not saying you are wrong for yours, it's just something I don't completely understand.
I have been an asshole who has had a rampaging ego in the past and preferred having my way through a large part of my life...but through several years and several experiences, I have had this beaten down into a much more humble shape. Personally, I admit it has been good for me..healthy. I am sure that too may be different for others.