Housing prices. Seriously.
Polyamory is on the rise, and divorces are at a 50-year low[0]. This isn't a coincidence.
Where a couple would have divorced in the past, it now is financially infeasible in many cases. Living with a partner you're no longer compatible sucks, but not being able to afford rent sucks worse. So couples are pulling out all the stops to be not-divorced, including looking for other partners and keeping a marriage-in-name-only.
[0] https://ifstudies.org/blog/the-us-divorce-rate-has-hit-a-50-...