Ha! That's probably way low on the list of concerns.
People asked why even have this cross your mind? It happens occasionally that 2 different men's cum going into a woman even a day apart, can throw her PH off enough that she can get a case of bacterial vaginosis, or BV.
This isn't common, but it happened to a polyamorous woman I dated, more than once. Most vaginas can deal with the changes and just ride out the mild storm of lots of different semen.
The bigger concern is HPV. If she is under 45 (or close to that), there's a vaccine that greatly reduces risks. Men are frequently silent carriers if hpv.
Otherwise, everyone having been tested sort of recently reduces risk tremendously for the other sti's.
I think doing a bareback gangbang once or twice in life is worth it. We all have different levels of risk tolerance.