Natural weapons do not use dexterity for attack rolls “naturally;” a creature needs to take Weapon Finesse in order to do that. For example, see the cat; it gets Weapon Finesse as a bonus feat.
Touch attacks are not natural weapons; they do not follow the rules of natural weapons, but rather of “weapon-like spells,” which are mostly similar to manufactured weapons. For example, you don’t automatically get one attack/touch-effect you have available, but you could conceivably get iteratives if you had a touch-effect that lasted, such as chill touch (you cannot with charnel touch because it is explicitly 1/round, but you could use it for one attack and then use your iteratives for attacks with other weapons).
You can treat touch spells as light weapons and use your Dexterity modifier (instead of your Strength modifier) on your touch attack rolls with such spells.
Complete Arcane pg. 73.
Unfortunately, the only way I know of for getting Weapon Finesse without taking the feat only works for weapons, by getting a feycraft weapon.