Ask about orgasmic energy.
From Create Your Own Story
You look at him innocently. "And how would you get that energy?" The Doctor looks shocked. "You know how. The..." His hands wave around. "You know." "Know what?" A cheeky grin appears on your face. The Doctor gulps. "I'm not saying it." "Doctor, we don't have much choice." You respond insistently. "No!" He runs over to you and cups your face in his hands, staring into your eyes. "I can't do that to you. You're just a girl." You stare at each other for ages. you can see him considering it. "I'll let you." you say, trying to hide your excitement. "For the good of... The universe or something." The Doctor gulps again. He hasn't even heard you. He's just gazing into your eyes. "You're so beautiful." He says dreamily. Now what?