Non-Monogamy Goes Mainstream: What a Belgian Study Really Reveals
A new study from Belgium is challenging what many people still think of as the default in relationships: monogamy. The research, published in October 2025, is titled Margins or Mainstream? A Cross-Sectional Study on Consensual and Non-Consensual Non-Monogamy in the General Belgian Adult Population and is available on ResearchGate. It suggests that non-monogamous experiences are far more common than most people assume, even in countries where monogamy still dominates social norms.
Key Findings
A national survey of nearly 2,700 Belgian adults found that:
• About 29 percent had been in an open relationship where both partners agreed to outside intimacy or sex (Belgian News Agency).
• About 32 percent reported a secret affair, fling, or other non-consensual form of non-monogamy.
• Together, these figures show that nearly half of Belgian adults have engaged in some kind of non-monogamous activity, even though about two-thirds still describe themselves as monogamous.
• The researchers conclude that monogamy can no longer be viewed as the unquestioned social norm.
• Younger adults aged 18 to 25 were more likely to report consensual non-monogamous experiences (20 percent in the past year), compared with only 5 percent among those aged 60 and older.
• Differences between men and women were small, but non-heterosexual participants were more than twice as likely as heterosexuals to report non-monogamous relationships.
Why This Matters
At PTC, we see this as more than just a set of statistics. It highlights how relationship models are changing, and how professionals who support individuals and couples need to adapt.
1. Recognising diverse relationship structures
Although monogamy remains the most common self-definition, the gap between identity and experience shows that many people are experimenting with or re-defining what commitment means. Therapists and educators need to speak about multiple models of intimacy as part of the normal relational spectrum, not as marginal or unusual exceptions.
2. Understanding consent and communication
The Belgian study makes a clear distinction between consensual and non-consensual non-monogamy. This difference has major implications for wellbeing. Whether a relationship is open or monogamous, communication, negotiation, and emotional accountability are what sustain trust. These skills are not unique to non-monogamy. They are the foundation of all healthy relationships.
3. Recognising generational change
Younger adults are much more likely to have tried non-monogamy with consent. This points to a generational shift in how intimacy is approached. For practitioners, it means updating our assumptions and language. We should not assume that clients automatically value traditional monogamy, nor should we assume they reject it. The goal is to understand what works for each person or couple and how they navigate meaning, trust, and desire.
4. Implications for therapy, education, and policy
Training programs for therapists need to include practical education on non-monogamous dynamics, including negotiation, boundaries, jealousy, and relationship transitions. Relationship and sexuality education in schools should also include clear information about consensual non-monogamy. Policy makers may need to consider how laws and systems recognise family and partnership models that are not limited to a single dyad.
A More Honest View of Reality
Monogamy is still the most common way people describe their relationships, but this study shows that real lives are more complex than labels. Many people who call themselves monogamous have experiences that fall outside strict exclusivity. For practitioners, this means asking open questions about what people have done and what they want, rather than assuming their structure or values.
The Broader Conversation
If almost half of adults in a country like Belgium have experienced some form of non-monogamy, perhaps it is time to stop treating it as niche or deviant. Instead, we can recognise it as part of the ordinary range of human relationships. For those working in therapy, education, or health, this means embracing relational diversity, encouraging consent-based communication, and supporting people to form relationships with honesty and care.
Mourikis, A. et al. (2025). Margins or Mainstream? A Cross-Sectional Study on Consensual and Non-Consensual Non-Monogamy in the General Belgian Adult Population. ResearchGate link.