The presence of inter-ethnic consensual unions (i.e., marriages and cohabitations) is an important indicator of social closeness between groups. In Europe, previous research has focused on unions between ethnic minority groups and the majority population, highlighting that religion, especially Islam, constitutes a bright boundary. However, in diversifying societies, opportunities for union formation between distinct minority groups are increasing. Yet, we know little about which ethnic groups are partnering with each other, let alone whether shared religious heritages promote inter-ethnic unions between groups. In this study, we thus examine union formation between any combination of the largest 21 ethnic groups on the partnership market in the Netherlands using full-population register data. The findings of our conditional logit- and meta regression analyses reveal that inter-ethnic unions involving partners from different Muslim-heritage groups remain overall strikingly rare compared to ethnic endogamy. This indicates the persistence of ethnic legacies rather than exclusively religious closure. Across Muslim-, Christian-, and secular-heritage groups, inter-ethnic union formation occurs merely between groups originating from neighboring countries, highlighting the role of pan-ethnicity over religious heritage. Overall, these results underscore the need for a more nuanced understanding of religious boundaries in union formation in Europe.