Well, Rory, I figured you could Google "open marriage success rate" as easily as I did, but apparently not.

They seem to all site the same study, and there isn't much research since most statistics show only about 1% of couples are engaging in it. Most participants are young (20s and 30s) which seems to indicate that once relationships are more mature, they no longer opt for this lifestyle.

https://www.srrentzlaw.com/blog/divorce/does-open-marriage-affect-divorce-rates/

https://www.emotionalaffair.org/do-open-marriages-work/

https://www.psychologytoday.com/intl/blog/magnetic-partners/201307/the-slippery-slope-open-marriage

https://www.bawdybeauty.com/blogs/the-bite-blog/pros-and-cons-of-open-relationships