Shiva, they say, is so innocent that he does not know the difference not only between woman and wife, but also between man and woman. And so when Vishnu takes the form of Mohini, he intimately hugs Mohini right in front of his wife. Both Vishnu and Parvati do not know how to tell Bholenath that such a display of affection is inappropriate in culture.
As Mohini, Vishnu was female, so Shiva hugged a woman.
