Could you do a transformation, where the He is geting turned into a women by his Girlfriend and his girlfriend would grow herself a cock so she can do the penetration. And he gets fucked by her?
... looks kinda like, contrary to her better wishes, Alison is learning to love the taste of cock as it looks like she's dragging her tongue ever-so slightly across the bulbous head of her male paramour' (sp?) manhood!!