You can pivot, but don't react to it with a pivot, make sure you get the lateral step to angle off and pivot once their side kick has missed. Also - a BIG thing is to make sure that if you step outside of them (to take their back as it were) , tie up fast or attack quickly with a solid guard - because if they are smart, they will spin a back fist after a failed side kick, and you can wind up catching a forearm or elbow to the face
So if you're fighting
@exclamatio and he side kicks you, scoot out towards his back, so left side kick, you go to your right, and vice versa - fire a straight punch as you're stepping out, THEN you can get a pivot or a tie up.
It should look exactly like this, but red trunks through a side kick before that rear hand and pivot