Ask for a hand.

"Why don't you help me fill it." You say, taking down your pants, sitting on the edge of the bed.

"Sure, I'll help." She says, moving towards you. She pulls down your underwear and begins stroking your dick in a calm professional manner. She smiles at you while she does it. You squeeze her tits to help you along, and she barely reacts. "Let me know when you're about to ejaculate so I can get it in the cup."