It's your body... not hers. So if you want to use a condom you can (and should). It's not a democracy when it comes to birth control. If you want to use it, then use it.
Unless one or both are sterilized, pregnancy will always be a possibility... and condoms work well by themselves... and when combined with other forms of birth control they help tremendously.
There are enough "whoops" and "oh shits" in the world... don't add to it.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen