Unless she is the rare exception, didn't your wife fuck guys before she met you? Maybe even a little overlap between the time you guys first started fucking and when she gave up her previous sex partner? How did you live with that image in your mind?
Maybe as a first step to condition yourself to the reality of the world that people, including women, including your wife, want multiple sex partners even after they get married, you two need to talk about her sexual past. Start with just asking her who (other than you) she fucked the most. Digest that thought. Days, weeks or months later ask her who she enjoyed the most (other than you), have her describe the experiences, what she really liked about her favorite sex partners. Eventually, you will not only get used to it, but also come to like the thoughts of her with another guy and take joy in the thought of her being pleased by someone else. Then slowly move on from there.