I'm not suggesting the US or UN become active participants in the Libyan Civil War. I can see both sides of this including that there would be considerable friendly fire accidents, we might have to choose between factions if the rebels splinter, and the last thing American needs is to get involved in another Muslim war. But the question is why is the "gung-ho" military so reluctant?
It Would Be a 'Military Operation'
This is what passes for an official reason (To which the appropriate reply is, "Duh! That's your job."). The Pentagon is heavy laden with bureaucrats and can't do anything without months of planning. This inflexibility has been a major problem in Afghanistan.
They Are Scared
The last air-to-air combat for a US pilot was over Yugoslavia in 1999. Sure, the current generation of pilots have flown lots of sorties (drilling holes in the air) and bombed defenseless ground forces but aerial combat is a lost skill.
It is well known that Gadhafi's fighters are mostly highly paid mercenaries. What is not known is where is he hiring these mercs. The best paid, best trained soldiers of fortune are Americans. It is possible that Navy and Air Force commanders know that their old buddies and bunkmates are flying Libyan planes and and they don't want to endanger their friends.
Also possible is that Blackwater (Xe) has quietly told commanders to stay out of the fight and that anyone attacking mercenary forces in Libya will forfeit any chance of future employment in the lucrative field of private warfare.