I have seen one nutfight online. Check this post, third video down:
Cockfight Club: Cockfight : Cockbattler91 vs Anonymous
I like one concept that you mentioned when you described nutfighting: the idea of mutual damage. In my own experience with cockfighting IRL and my fantasies with both FvF and MvM sexfights, for me the major appeal has always been the concept of mutual "anything". Whether pleasure or pain, the idea of taking the most sensitive parts of your body and then slamming, ramming, slapping, and grinding them together with someone else and feeling that mutual pain and pleasure at the same time has always been a huge turn on for me. Sometimes you read or see media with sexfights or cockfights where the competition is not symmetric, more of the no-holds barred routine where any advantage you can get you will use, but that has never drawn me in like mirror matches where the competitors actually try to time their thrusts or blows so they land together for more impact.
I definitely appreciate the notion that pain is an under represented feature of sexfights and titfights, but I think that may have to do with how highly sexual the fiction is intended to be, and there is a very real pain threshold where it is not even remotely pleasurable, but we like to see more aggressive fights in our fiction, in part a desire for a more intense experience in that fantasy. The IRL experience is always a bit tempered from fiction, for one, safety, two, boundaries with another person, and three, what is actually enjoyable even knowing that pain is a real part. The intensity is going to be a bit less than in fiction, and certainly the pain threshold of anyone is going to be exaggerated in fiction. For that reason I think a nutfight may be very rare IRL.
I can imagine titfights of any variety are painful, and certainly cuntbusting is another area where fantasy is extreme and reality is tempered in intensity, but it is hard to stress how any pain in the testicles can be truly sickening and immobilizing. I'm not sure there is any way to describe it without experiencing it. I think most other forms of erotic competition or even combat have large influences of knowing pleasure can be experienced even under a painful sensation, but testicles are an area where I think a lot of guys would have a hard time seeing any possible pleasure in a nutfight because of how poorly that particular part of the anatomy responds to pain. I get that some guys like CBT, but I can only imagine how slowly they have to build up to find their limit otherwise you could have only one good slap that turns out to be over the top and then all possibility of enjoyment is over for the rest of the night while you take aspirin and ice your balls. It seems like it would be hard to get there safely in a competition.
I do understand the appeal, and when it comes to grinding and thrusting cocks together in fantasy and IRL I like having the nuts press and bump together as well, but not with overbearing force. I think more realistically you may see it either standing or kneeling and instead of having a bearhug, you grab each others asses and pull your crotches together as hard as you can. That is much more bearable than an immediate strike on your balls, so there is more room for competition and enduring more than your opponent. You could scissor legs and bring your balls together, but I really have to stress how slowly that competition would need to start out. In a lot of fiction you even read about female antagonists really putting all of their muscle and weight behind a move or thrust, but in this case that would probably knock both guys out from pain and likely end up with a hospital visit. In life and in fiction I think it would make sense to slowly, slowly, slowly, increase the force and tempo with any kind of nutfight. Like an escalating game of mercy instead of outright combat from the start.
One thing that I think gets missed a bit is how irritating and additionally painful erotic competition can be without any form of lubrication. Two dry cocks slapping and ramming together is probably just as bad as two pairs of tits slapping and ramming together in terms of getting skin burns from the friction. I would much rather see and feel two lubricated cocks or pairs of tits collide. The motion is more fluid and the sound is absolutely delicious. Also when you both actually time it and aim well you get an even more satisfying impact because there is no friction keeping cocks or tits together, so when you have that kind of mirrored collision and the parts stick, it feels even better and you know that both you and your opponent felt that same "mutual damage".
Right underneath the head of my cock where it meets the shaft is where my cock is the most sensitive, like where the crown of the head wraps around and meets under the bottom - the frenulum. When it comes to cockfighting, my favorite activity is to line that up with another nearly identical cock and then time it so we both slap them together at the same time. The idea of matching up the most sensitive and pleasurable part of your entire body and then forcing it into a mutual collision of pleasure and pain with the same most sensitive spot for another person has always been the key part to my desire for cockfighting and erotic competition, like any mirror match nipple to nipple or clit to clit. I guess where I struggle with the balls to balls match up is that the pain threshold is substantially lower that I find it hard to derive pleasure from the concept. It is still interested from a fictionalized version of MvM, but you may not find it much IRL.
As for what would drive me to "fight" another male, I would say in fantasy clearly a girlfriend, wife, or simply any object of mutual desire, or in fiction you can do something as basic as that one point of eye contact where both guys simply "knew" that they had to fight. IRL I don't think you find that kind of organic progression from a conflict over a woman or a specific meeting that leads to a cockfight, you only really find what you are looking for after a pretty rigorous search and then vetting of anyone you want to meet because you are pushing the limits on personal boundaries and safety. You also need to be pretty closely aligned on where you and your partner are on the Kinsey scale. If found it is hard to get the right guy who enjoys both the competition and mild pain in a cockfight as well as the pleasure with another man in a first to cum contest. Like towards the heterosexual end of the spectrum I find there to be a lack of desire for mutual pleasure during a first to cum contest and it even removes that as a possible activity, and on the homosexual end of the spectrum there is not enough desire for the rough contact of a cockfight, it seems like more of a funny and particularly poor form of foreplay, like when the female actresses in any titfight or sexfight laugh a bit too much, there is no competition or at the very least intensity.
