[mkaras]

The standard gate devices are designed according to the logic function desired. So it is not the gate that is "naturally" inverting.

If one goes to the basic level of the transistor circuits inside a gate design you will see that the logic of a single stage of a CMOS device is an inverting structure. Some of the desired gate functions of standard logic packages will be able to be implemented with the these single stages and so would end up inverting because of that.