Why not turn it over to her?
OK. It was very easy to interpret what you wrote in another way. You didn't explicitly mention the "shit happens"-perspective, which makes it very different. And I wasn't the first one to interpret it that way.
In that case, why not turn it over to her? What kind of safety level would be good enough for her. Let her define the standards. For example, if you always use condom AND abstain from ejaculating inside the other partners, the likelihood of conception is practically nil. Dont try to tell me something else. The same would apply for most other problems. Make her do some simple probability calculations, and compare with other risks in daily life, would bring out clearly how ridiculous her fears would be. My question here is: Would you be willing to let her set the rules in this way, subject only to adjustments from negotioation?
And what would YOUR position be? If, in spite of all precautions, a partner gets pregnant without you being technically to blame, it turns out you are the father, and she refuses to have an abortion.