(I mean no disrespect to Craig by carrying on a disjointed conversation with a thread obviously happening elsewhere, I just have little desire to interact in that other forum).
1. The argument from the New Testament.
The absence of MMA and related issues in the NT is inadequate in this case, for its justification. That is, I'm well aware that arguing from the silence of the scriptures is a more difficult proposition. I don't need to say "The NT doesn't support X, Y, Z, therefore they are not appropriate". What I am instead saying is, "The NT is overwhelmingly against the use of violence, therefore the transformation of violence into entertainment is also suspect."
The question then becomes, is the NT overwhelmingly against violence. I tend to make sweeping statements on this point, but only because I think part of the argument needs to be the broad sweep of the NT canon in general. More specifically, consider (a) the Sermon on the Mount, (b) the spiritualisation of conflict in Ephesians 6 (Eph 6:12 especially), (c) the expectation in John 15:18-16:4, (d) the call in Mark 8:34 and its parallels, (e) the pattern of 1 Peter, especially 1 Peter 2:19-25, (f) Romans 12.
Each of these strongly portrays a NT pattern that the ethical life of discipleship is marked by a persistent imitation of Jesus, not just in general, but by conformity in humility, non-self-aggrandisement, service, and ultimately suffering and death. It's not just persecution that's being put up here, but the refusal to achieve victory through force, which is the very pattern of the cross.
Revelation might read as the great counterpoint to my argument, but it actually strengthens my case time and time again. Here is the great finale of God's plan, the judgment that makes sense of both OT and NT, and every time Jesus is portrayed as a conquering warrior king, the text works to subvert this notion by pointing to the victory through the Cross. The contrast between the Lion of Rev 5:5 and the Lamb of 5:6 is paradigmatic of what I'm talking about.
Lastly, let me deal with Romans 13, albeit briefly. Here Paul endorses secular authority's right to execute judgment. In no way do I dispute that, I simply note three facts, (a) Paul does nothing to legitimise state-sanctioned violence in a participatory fashion for Christians, (b) Romans 12:14-21 must not be divorced from any reading of Romans 13, (c) Romans 13 adds little to a debate about war, let alone self-defence, and very little at all to violence as entertainment
2. Emotional rhetoric
I do tend to use a bit of emotional rhetoric on this issue. If that is clouding my arguments, I apologise. Let me only note that I seek to appeal to the visceral nature of the sport, and in doing so combat its own very visceral appeal.
3. The weight of argument
It's not simply the case that "if there's nothing against it, it's okay". Rather, we must exercise caution in prohibiting what the Bible does not prohibit. Clearly I agree on that, since I find the Regulative principle in worship so appallingly sub-biblical. That said, the weight of my argument is this: the NT opposes violence, and it's very difficult to make a pro-violence argument from the NT. It's not that the NT has nothing to say about violence at all. If that were so, we might be in a different situation. But it's not silent, it's very clear that violence is not an appropriate means for a disciple of Jesus, because Jesus himself rejected it. If that is the case, what step justifies legitimising it as entertainment?
4. The OT?
Before I leave the matter, some well ask what about the OT? My own take is this: OT violence is put into a paradigm of God's sovereign justice, judgment, and care for the world. In the NT that is (a) taken out of the hands of God's people, (b) dealt with at the cross, and (c) relegated to Jesus' return and the final Judgment. That's part of the reason I think a NT appeal is sufficient on this issue.