Nyotai-ka.

Manaka Makoto, 24 years old and a salaryman without a girlfriend, wakes up one morning suddenly changed into a female, and a knockout at that who attracts just about everyone of the opposite sex. And when he learns through a magical being that the only way he’s going to become male again is by getting aroused (as a woman) he tries to find a way back without any success. But when success finally comes and he switches back, he discovers that he will switch back and forth ANY time he gets close to someone sexually, and there are exceptions…

(Source: Animeraider)