https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.789983/fullDefine "problem". They do experience a pathogenic (reduction in l.crispatus, introduction of penile anaerobes) change after each intercourse, it's just that it does not always produce short term symptoms. Diagnosable BV that makes you smell foul and causes pain, iirc, is found in 30% of women, subclinical BV-liks microbiota is more common, and restoration of original l.crispatus microbiome is possible if you don't have sex for X amount of time. It's fair to assume that the initial penile microbiome transfer/reintroduction will cause your vagina to hate you unless some sort of immune tolerance is involved eventually, which could explain the vague "BV may be caused by a change in sexual partners" idea. Interestingly, female chimps and bonobos all have BV-consistent microbiomes, while humans don't. The native "healthy" microbial makeup of a vagina is also mildly spermicidal kek.
BV is independently
triggered by the male anaerobe-dominated microbiome (hence why you can get it from a virgin), but a specific strain can also be transferred by a male from another woman.
So in conclusion, women in long term relationships do have "problems" or a deviation from an objective standpoint, they just may not personally know, think it's a problem, or their microbiomes are made to be less hostile by BC. For example, if you were to decide to have regular sex with him, the natural state of your vagina would change and you would no longer experience symptoms after every intercourse. Also, uncircumcised dicks have
a lot more anaerobic and fecal bacteria, so your vagina is giving you the full "what the fuck did you just put in me I fucking hate you" experience. It will give up eventually.