Blue Head ni Kuchizuke

Rei Sunada is the secretary to the president of a major cosmetic company. One day, due to excessive stress, she developed "gender reversal syndrome," a condition that caused her to change into a man. The only way to revert the change back to her female body is to have sex. It turned out that the president of the company, Kanzaki (♂), also suffered from this strange disease...? In order to return to their original bodies, the two are forced to have sex with each other...!

(Source: Bébé, translated)