Pegging

Many heterosexual men are disconcerted by the thought of a woman with a dildo. For not only can she penetrate herself with such, but she can penetrate them. Only with great reluctance would they submit to a partner with a strap-on who challenges conventional notions of who does what to whom. 

Their unease is related not just to the homophobic fear of accepting a cock into their anus (albeit a rubber one), but also to the wider concern with passivity which, in the male mind, is often thought of as humiliating and castrating. 

It's an old problem: one that greatly troubled the ancient Greeks. But Sade laughs at such moral anxiety and prejudice, insisting that not only are both classes of intercourse - active and passive - perfectly legitimate, but that it is the latter which ultimately affords the greatest pleasure: since one enjoys at a single stroke the sensations of before and behind.