This excellent piece from the International Business Times explains some of the possible roots behind this statement, including quotes from the Tibetan Feminist Collective (which you should be aware of if you’re remotely interested in Buddhism and Feminism - they’re amazing):

Gelug monks (of which the Dalai Lama is one) were responsible for removing aspects of certain rituals in which female mystics were invited into monasteries. They declared that it was equally, if not more beneficial to visualize imaginary women during these practices.

In other words, Dad joke or not, the Dalai Lama’s…