It's quite simple really. Just read the prerequisite. A monk's unarmed strike is not a natural weapon. You don't get the benefit (i.e. the effect of the feat) until after you take it, so the line about spells and effects in the monk's unarmed strike description are really useless.
When deciding if a monk can take the effect, look at the relevant part:
Is the monk's unarmed strike actually a natural weapon? No. At this point, there is no 'effect' to equate it to such, so you stop and say that the feat doesn't apply.
The defense rests.