Can you be legally married and practise polyamory?
Yes. Legal marriage and polyamorous practice are not in conflict. Many polyamorous people are married to one partner and have additional partners outside the legal marriage, with that partner's full knowledge and consent.
Legal marriage is a contract between (in most jurisdictions) two people that allocates specific legal benefits and responsibilities. The contract does not require sexual or romantic exclusivity — those are cultural expectations that often accompany marriage but are not legally encoded into it.
A polyamorous married person typically maintains the legal marriage as a practical infrastructure (taxes, healthcare access, immigration status, parenting authority) while also being non-monogamous in practice. The non-monogamy is fully consensual: the spouse knows and agrees. The legal marriage doesn't dissolve, doesn't need a special carve-out, doesn't trigger anything.
What does require attention is what the marriage is being asked to do. The cultural expectation that a marriage is the central romantic-sexual relationship, with all others excluded, is the part that needs to be re-examined and explicitly re-negotiated. Many polyamorous-married couples describe an early conversation in which they distinguish the legal marriage (which they keep), the cultural script around marriage (which they decline), and the relationship itself (which they actively redesign).
Practical risks for the married polyamorous person include: outside-of-state recognition issues if they get involved in jurisdictions that view structure unfavourably in custody proceedings; insurance complications if a non-spouse partner needs coverage; the partner-status asymmetry in which the legally-married partner gets recognised authority (medical decisions, etc.) that other partners do not. Most of these have workarounds — healthcare proxies, durable powers of attorney, explicit wills — but they require deliberate setup.