The unchanged argument.
The reason Christians should not watch MMA or similar sports, remains the same as the reason Christians of the early centuries avoided the gladiatorial games. It’s a reason grounded in the NT and the NT Ethic.
When one reads the New Testament, one reads in vain looking for any Christian case legitimising violence. Instead, gospels and epistles alike depict a Jesus who refuses to utilise violence as an option, and calls his followers repeatedly to follow that same pattern. This is further emphasised in the epistles, which present Jesus as an ethical paradigm, and in revelation, the most militant of NT books, which spends much of its imagery of reconfiguring apocalyptic expectations around the victory that is through the death of the Lamb.
There remains little doubt, historically, that this kind of Jesus-centered pacifism was ubiquitous in the early church, and it extended to the moral prohibition of enjoying gladiatorial games.
[Even if, you derive a just-war doctrine from the scriptures, the very basis of just war theory is that violence is a tragic necessity of a fallen world, not a glorious means of changing it. Far too many assume a doctrine of just-war, but actually live with a doctrine of militarism, if not crusaderism. The connection I am about to draw is not particularly weakened if you think Jesus permits self-defence.]
What’s different today? In some ways much: UFC and the like are not sports unto death. They have a certain restraint. The entertainment is not in blood and guys, but in bruises and pummeling. And yet the spectacle is still unadulterated violence, still two men stripped of their human dignity and put to play for others’ enjoyment, still the cheapening of humanity in the divine image.
Make a sport of it, professionalise it, and put it on tv (objectifying it and further distancing ourselves from it), doesn’t change what it is. Do we tell our sons to gather round fights in the school yard and cheer them on, knowing that that too is mostly spectacle? distance and objectification remain some of the most powerful de-moralising factors of our mass-media world (I will spare you the parallels with porn). What is immediate and experiential is grotesque, what is absent and vicarious is fabulous.
Consent is not the deciding factor here. That two human beings consent to degrade one another for my enjoyment doesn't change their fundamental act.
Let's just cut to the conclusion: MMA and the like are the glorification of violence as spectacle. Do we really think that is 'ok' as Christians?