Seme has a sex friend but only uses him as a substitute because he can't be with the one he really loves, his best friend. The two of them (the best friend and his sex friend) get into an accident and switches bodies (actually the sex friend is lying about switching, he just wants the seme to love him and not just use him. The seme ends up falling inlove with his sex friend. Who apparently has loved the seme since high school
Kakkou no Yume