I just don't get this cuckold/hotwife thing. I just don't. I guess I'm not that highly evolved.
I couldn't watch the woman I loved being railed by another guy.
Bandit,it's actually the exact opposite. The act of another male having your female stimulates a region of the brain referred to as the limbic system. It's function, in our evolution, was to cause strong motivation prompting the male to stop another male's advances and then reaffirm his dominance by inseminating the female in question demonstrating his virility.
This drive is very strong in most males which is the reason that it works when other forms of "arousal" do not. As evolved beings, we now have the intellect to assert logic and reason over the instinctual drives in our brains but not always.
With logic and reason it is easy to understand that this form of "arousal" is fraught with peril to monogamous relationships because of the high possibility of unintended consequences. It is playing with fire because the female instinctual drive is to find the "best" male specimen in order to provide the highest probability of producing viable offspring. If she is lacking the intellect necessary to fight those urges, another male can, at any given time, become more appealing than the current one.
In any event, the issue here is not this instinctual drive but rather the fact that her H cannot use logic and reason to squelch it in light of the fact that she does not care for it. His instinct is overriding his intellect. This could end very badly not only because of what I stated above but also because the OP does not like it and is trying to make him understand this. His failure to do so may cause her to consciously, or subconsciously, sabotage the relationship.
You should discuss this with your H and tell him that this makes you uncomfortable and is not something you enjoy. You have already said that he has agreed not to do it so leave it at that. You are not "depriving" him by refusing to do what makes you uneasy. Is he allowed to sleep with whomever he pleases? Are you depriving him of that? Of course not, that is just part of being in a mature relationship with someone you care about.
Also, with all the information available today with a little effort and research on your part you may find alternative sources of arousal that work almost as well and that you are comfortable with.