How can omission be lying? If the question asked is: did you have sex in the car? And the answer is "Yes" but the given answer is "no"...then yes a lie has occurred. If the questions is: did you have sex? And the answer is yes but you do not give the details well then it is omission. If the request is, "I would prefer you didn't have sex in my car." And the answer is, "Okay I won't have sex in your car." Lying would be if you have sex in the car after
the request was made; omission would be the thought: "Well I already had sex in the car, telling her will annoy/upset/cause an unnecessary meltdown because there is no erasing history but I will not have sex in her car in the future."
That's my opinion. Not a fan of the open-ended question, why ask them if you don't want definite answers? (Wait...that's an open ended question! Scratch that opinion!