The study of size-assortative mating, or homogamy, is of great importance in speciation and sexual selection. However, the proximate mechanisms that lead to such patterns are poorly understood. Homogamy is often thought to come from a directional preference for larger mates. However, many constraints affect mating preferences and understanding the causes of size assortment requires a precise evaluation of the pair formation mechanism. Mate-guarding crustaceans are a model group for the study of homogamy. Males guard females until moult and reproduction. They are also unable to hold a female during their own moult and tend to pair with females closer to moulting than them. Using a theoretical approach, we tested the potential for size-assortative mating to arise from such a state-dependent male decision rule. Consistent with previous experimental observations, we found a pattern of size assortment that strengthened with maleemale competition over females. This decision rule, which we call the female-sooner norm, may be a major cause of homogamy in mate-guarding crustaceans. This highlights the potential for size assortment to arise from preferences not based on body size. It emphasizes the importance of considering pair formation processes when studying the link between preference and pattern in order to avoid inferential fallacies.