Contamination implications of eating peanut butter with a spoon
In addition to actually adding unwanted species, you're also adding moisture to a food that's otherwise very low in water. This will change its ability to support the growth of mould and bacteria in the damp bits.

If you polish off a jar in a few days and always eat the bit you previously got saliva on, I'm sure the risks will be vanishingly small. If you leave it for a couple of weeks after dipping in a well-licked spoon, I'm less confident.

As a practical measure it would seem easy to spoon out as much as you want into another container, then eat that. That also means you have the option of sharing it rather than having to refuse on the basis that you might as well have spat in it.