<p>Non-monogamy refers to a type of relationship wherein sexual and/or romantic exclusivity is not upheld. While scientific research has been increasingly focusing on the prevalence of non-monogamy in Western societies, studies typically suffer from methodological issues. In the current study, we investigated the prevalence of all forms of consensual and non-consensual non-monogamy (CNM, NCNM) as well as personal values and norms related to non-monogamy in a sample of 2691 Belgian adults, representative of the general population. Although two-thirds of the respondents stated that they viewed themselves as solely capable of monogamy, approximately half reported having engaged in some form of non-monogamy at least once. Approximately one-third of the sample reported lifetime experience with CNM, and a similar proportion reported lifetime experience with NCNM (not necessarily involving the same individuals). Additionally, 16% indicated non-monogamous experiences in the past 12 months, with comparable rates for CNM and NCNM. The prevalence of lifetime practices of CNM was equal in all age groups. Contrastingly, numbers for CNM in the past 12 months, including polyamory and consensual prostitution visits, were significantly higher in the younger age groups (19.5% for the youngest and 5.0% for the oldest age group). No gender differences were found in reported lifetime experiences with non-monogamy, but men actively engaged about twice as often in non-monogamous interactions (both CNM and NCNM) in the last 12 months. Non-heterosexual individuals reported more involvement in all forms of non-monogamy compared to heterosexual respondents. Education level had no impact on the prevalence of any form of non-monogamy. Our findings suggest that non-monogamous practices are relatively common in the general population and may reflect increasing diversity in relationship structures, challenging the exclusivity of mono-normative beliefs in certain segments of Western societies.</p>