How to say no to sex? Huh, the chance would be a fine thing.
I have never experienced this dilemma in all of my years on this earth, so I'm probably not much help. All I would suggest is that sex, and all the stages leading up to sex, require volition. In other words, it's not something that 'just happens', it requires your consent. So, you have ample opportunity to decline, but it's entirely up to you how you exercise your own will. I'd also suggest that temptation can highten the more stages along the road to coitus you travel, so if you're serious, you ought to be saying 'no' long before she's inviting you into her bedroom alone.
Then again, I've sat with lass in her bedroom for hours drinking cider. Nothing happened apart from some amusing Chevy Chase films, but as I said, I appear to be free from this affliction of exciting sexual desire in women wherever I pause. Perhaps I am to envied.