I think there are several reasons. On the one hand, Zhang Wuji is lewd. Otherwise, he doesn't have to take off this woman's clothes when conveying internal force. And women are embarrassed to say, after all, he is helping himself heal, and can only accept it.

On the other hand, it can better transfer this internal force to women. After all, men and women learn kung fu completely differently.

And different in nature. Men are masculine and women are feminine. So in the process of transmission, it is more important to master the accuracy and precision of this internal force. If there is deviation, it will lead to the woman's internal force transmission is not in place and can not heal better.

In addition, women will have more clothes, and women will have this underwear, which may be blocked when you transmit internal force, so be sure to take it off.

Men, on the other hand, are masculine, so in the process of transmission, everyone has the same internal force and there is no need to take off their clothes. Moreover, it is quite troublesome to undress, and direct transmission is more practical.