If my girl would only follow the rules i make for her, hehe, this would be one of them: No sexual stimulation (for both of us) within 72 hours prior to a date! This helps build up the sexual tension and keep the anticipation high, also makes the essential event more intense and provides for a better relief afterwards.
Usually, men cum faster when they are highly aroused. Knowing he has a woman who is not prudish sexually but lives her desires may be raising his arousal and help him cum faster. See it as a compliment for your sexiness!