Zenbu Zenbu Kimi no Sei

There's a “transsexual syndrome” where one's heart fluttering causes one's gender to switch.
Sagimiya Ryou and Mikami Nagisa, both possessing this condition, find themselves drawn to each other and become lovers.
Several years later, Ryou, now a university student, and Nagisa, who has become a teacher, begin their long-awaited life living together, spending their days all lovey-dovey.