Are you asking for a lie or an excuse? Huge difference.
Any excuse will work, because all excuses are just that, and being just that, are lame.
Lies only work when you get yourself to believe them first. That's how really good liars lie really good - they believe it.
Give her every signal that you really want to, but be sure to talk about all the strange sexual practices you're into. That works sometimes, however when it backfires - you pick things that she's secretly into also - it really backfires.
Or talk a lot about serial killers and how you in some ways really admire them, things like their planning and level of dedication. Desperate yes, but highly effective. Bonus is that in order to pull it off you have to research serial killers and that's pretty trippy stuff. Downside is that people start seeing you as a psycho.
How do the other guys on your dorm floor or frat tell you to do it? What's the secret 'guy advice' way to blow some chick off? How do you do that without either a lie or an excuse?
You play it cool boy, real cool. Beyond cool. Cold. (Like it doesn't matter and like you don't care). Of course she wants to have sex with you. What chick doesn't? So if you got all the girls in the world after you, you don't have to be in any rush to do any of them - all good things in all good time. The key to this is making sure she dead-to-rights knows that you absolutely don't care much one way or the other. Just keep thinking (and make no secret that you are thinking it all the time): "Sure baby, we'll get around to it but what are you going to do for me first?" I will guarantee you that attitude will prevent any guy who is not Brad Pitt or George Clooney from getting laid. It's about constructing a backdrop that what guides your thinking and your sexual choices is: Of course she wants me - everyone does, but is she really good enough for me - and damn few are --- and really I'm not all that into getting tangled up in any of that needy relationship bull->-bleeped-<- at this point of my life anyway, I really dig my freedom. And then making sure she knows that's what you're thinking and how you're evaluating romantic potential (use the words 'golddigger' and 'needy/clingy/suffocating' when talking about women/relationships). Talk a lot about the petty negative features that 'really turn you off' of highly desirable women. Pretty much that stuff will kill every romantic notion known to women in about 1.3 seconds once they see it.
Or just tell her that you think her mom is the first real MILF you've ever seen. That one is damn near foolproof.
