This is found in the body naturally, and can sometimes overgrow, and sometimes it can be sexually transmitted when that happens. It's not an indication that any one has cheated, which would be a natural question, but it's not like chlamydia or gonorrhea that requires an exposure to get.
There are no definitive standards to partner treatment, but some experts do recommend that partners be treated.
https://stdcenterny.com/ureaplasma-testing-treatment-prevention-nyc.html
https://www.medicalnewstoday.com/articles/321636