my 2 cents about the lance debate:
The rules on 3-to-1 power attack mentions two-handed weapons (a category) without mentioning the amount of hands this two-handed weapon should be wielded with. It also mentions one handed weapons (different catagory), but only if these one handed weapons (category) are wielded with two hands. This qualifier (must be wielded with two hands)is not added when talking of two handed weapons, so any two handed weapon gives 3-vs-1 on power attack
A lance is a two handed weapon, even when wielded with one hand, so it gives 3-vs-1 power attack. That's RAW.
However, this isn't the subject of the topic.
One more time: realism should not be a big consideration.
It does not really disrupt the game in ways unmanagable in terms of damage (we have lance for that...), he does not suddenly outclass the other party members. It takes investment (ride skill point) to really make it work, so there is some balance, and unless he has a special class-feature pet his mount is easily killed.
You also don't have the rules to back you up, the mounted combat rules allow it.
I think you don't have a legitimate reason for disallowing it. If you decide to anyway, I would call it picking on people.
