I know I am a girl, and I use the "I have to get up early" excuse. The real reason is that I usually am just not into him at that particular moment. But I think I may need some new excuses. LOL.
While I try don't engage in this, I do have a variety of excuses to use. A: If you've ended your cycle recently, tell him that you're still on. If he raises a brow, tell him that it's been really irregular this month and you don't know why. He should definitely hold his mule with that one. B: Fake a migraine or a headache. C: Tell him you're not feeling well and you feel nauseous. D: Pretend (If it's not natural) that you're really stressed over something at work and try to get him to comfort you instead.
Good luck. I hope I've helped!