Is a man allowed to use a flat iron to straighten his hair if the person doesn't like having curly hair?
kitzur shulchan aruch chapter 171 halacha 1
"It is forbiden for a man to wear even one garment that is designed as a woman's, although he is dressed in a manner that clearly indicates that he is a man." ... "This prohabition does not apply to clothes alone. A man is forbidden to wear or employ any ornament or beautifying practice that is unique to women in his particular community."
It would see such a feminine practice as straightening one's hair would be prohibited under the category of not wearing women's clothing...
Devarim 22:5 לֹא יִהְיֶה כְלִי גֶבֶר עַל אִשָּׁה וְלֹא יִלְבַּשׁ גֶּבֶר שִׂמְלַת אִשָּׁה כִּי תוֹעֲבַת יְהֹוָה אֱלֹהֶיךָ כָּל עֹשֵׂה אֵלֶּה
"No male article shall be on a woman, and a man shall not wear a woman's garment because whoever does this is an abomination before Hashem your G-d"