Ajoyful,
Based on science, I would agree with you. Cysteine is the amino acid that is responsible for making curly hair curly. It is this amino acid that is broken down when the hair is relaxed. The chemical in relaxer breaks the disulfide bones in cysteine. This is why relaxed heads especially want to have protein in their regimen somewhere, to provide some semblance of the amino acids that were broken down.
Heat also denatures the proteins in hair.--and straighter hair would have less cysteine than curly hair. This is not saying that straight hair does not have protein--all hair is made of keratin and therefore all hair has cysteine in it but straighter hair doesn't have as much as curlier hair. But I think the straightness of straight hair helps to keep it in tact. It isn't as "angular" and so it can be mechanically managed without a lot of pulling and pressure. Also, the straightness as we've always heard, allows the natural scalp oils to flow down the length of the strand. Because curly hair doesn't have those same characteristics, it does possess a fragile quality.
When straight hair is damaged by heat, initially it may be hard to tell because straight hair is already straight (it doesn't have any curl to lose). Heat damaged curly hair shows signs of this damage by losing some of its curliness. So with all that said, yeah I can see how your theory would be correct.