I think as a good Bayesian you actually DO have to view every member of your preferred attraction-category by sex as a potential romance, and every potential partner around your Significant other as, to some degree, a competitor. I often wonder if first order rationality is actually beneficial in this matter. I may instinctively trust my partner, but at what rate do trusting people get cheated on relative to non-trusting people? That's all the strength of evidence that my trust can offer me.