How do you stop obsessing about something perfectly normal? You let it become part of you. Yes, the media shows it everywhere, it is seen in every mixed couple you see. In advertisements, tv shows, and movies, more and more interracial connections are made. How can you help normalize it? How can you look at your wife and not see here milky white skin dominated by a darker larger companion? It's only natural. Can you talk to her, is this something she wants? Does she know? She chose you, she married you, how come she doesn't know you? Have you changed? Or is she ignorant of this media? Did you stop sharing your interests with her?
Communication is a large part of a relationship. If she's not happy, or gone quiet about her needs, you need to help her know she's free to be herself, to explore without fear of reprisal. She needs to know you wont run away if she ventures out. I've been locked in chastity to prove my devotion for over 6 yrs now. Be careful, and let her know what you know.