A BV researcher heard from a woman who started getting BV symptoms after being artificially inseminated at a clinic. This prompted the 2010 study (Swidsinski et al) on frozen donor sperm in Germany that found three out of the 20 semen samples contained cohesive Gardnerella vaginalis, the bacteria responsible for the biofilm that causes bacterial vaginosis.
There was some suggestion that IVF might have been a transmission method that needed further research, since G. vaginalis is sexually transmissible between male and female partners. Nobody thought to look in the freezer.
It is now understood to be a sexually transmissible infection, but it can also exist in those who are not sexually active due to the natural make-up of the vaginal microflora.