Chemical treatments, such as relaxers and coloring, thermal styling, and UV rays all lead to the softening of the keratin of hair. Any intentional process of altering your hair's natural texture is basically controlled damage to the protein structure of your hair.
Yes, relaxers permanently straighten the hair while thermal styling is meant to be temporary. Still, some naturals who've thermally straightened their hair had trouble getting their hair to revert and thus were left with straight sections of hair.
Yes, infrequent relaxers can be less damaging than frequent heat styling. Still "stretching" relaxers usually entails some method of heat styling which we know further weakens already soft protein bonds.
When I hear about a natural who has sworn off heat styling because of heat damage, many questions pop into my head: What was the temperature of the styling tool? Was it ceramic, tourmaline or neither? What was done to prep the hair? How often was she heat styling? etc.
Anyway, what's my answer? It depends! It depends on a number of factors ranging from hair type, to products, to frequency, to technique, the list goes on. What I do know, for sure, is BOTH can be abused and BOTH can be done safely. Everyone has to determine what works best for them. K?