1. The Libertarian Argument: The participants are not harmed in any way - and consent to being participants - not coerced in anyway. Viewing of the result is optional and avoidable. Since no direct harm is done in the production of this, and the results are not required viewing, there is nothing at all wrong.
2. The Paternalistic Argument: The results, even with full consent of participants, and with viewing being optional and avoidable, still will harm. Either by cheapening sexuality (exploitation of women, men, etc.), causing people to behave dangerously or badly (the porn causes sex crimes argument), or warp those that view it sometimes, excessively or at all. The result is both the fault of the producer, those that patronize, and those the commit crimes. Therefore is should be banned, restricted or regulated in some manner to protect society, and people from themselves.
I see these 2 groups going back and forth here. I do not think the bridge will be crossed here in APUG, but there is some sort of compromise reached in most countries. Some are more permissive (Amsterdam), some more restrictive (China, Saudi Arabia), some in the middle.