While on a dinner date, secretly jerk off into your hand. Wait until the girl gets up to use the bathroom, or distract her in some way. When she's not looking BAM! into her salad/food. Works particularly well if she's having fettuccine alfredo or some similar dish.
I was out to dinner with my girl last night. She got fettuccine alfredo, so when she wasn't looking I pulled a phantom gourmet and threw a fist full of cum in her food. She never figured out why the alfredo sauce was so salty.