"I have no reason to cheat on you. There's nothing another woman could give me that you can't. You're absolutely everything I want."Note that this won't work if she has an actual reason to not trust you - like, that you're ACTUALLY cheating on her. (And if you are, you're a douchebag and you deserve to get caught.) This will only work if she's being unreasonably paranoid and you want to shut her down. Don't try to argue with her or validate her specific points. Just look her dead in the eye and say it firmly, calmly and without blinking. If she doesn't calm down immediately, repeat - simply, logically, and matter-of-factly.
This line will accomplish everything - it's coldly logical, but it's also incredibly romantic. It compliments her deeply, while at the same time reminding her that she's an idiot for ever questioning you. You win.