I think we need some perspective. Is a head butt really that much worse than repeatedly punching someone in the head? Is it worse than punching someone in the side or back of the head? Why? Because its illegal in boxing? Where you are only allowed to use your hands? Because its against "the noble art of fisticuffs"?
If you want to draw the line somewhere, draw it at any player striking another in any way. Don't tip toe around based on your own subjective personal standards of what constitutes "ok" in a fight and what is "not on, mate, thats dirty fighting." If a player has intent to physically hurt or maul another player it doesn't really matter how he does it.
Luke O'Donnell deserved to get put on report and probably sent off for his dangerous tackle, and is a dickhead for it. But if you're not going to gvie the same crap to punch happy Thaiday for being third man in at least twice and and striking in the side of the head repeatedly than what's the point.
You wanted consistency, you got it. No players sent off for fighting.