It is both proper and necessary for lectures to be delivered often to the women of N'shei U'Bnos Chabad (see Hilchos Talmud Torah of the Alter Rebbe, conclusion of ch. 1). If this is better accomplished - and even if this is doubtful - by a male, then a man should speak (Pischei Teshuvah on Shulchan Aruch, Yoreh Deah end of chapter 240).

Excerpt from a letter of the Rebbe