Your first paragraph is not quite accurate. The high iron levels that occur during menstruation tend to retard growth of some of the problematic flora (although this is more true of yeast infections).
Sperm and blood can make BV worse as they are alkaline in the vagina’s environment.
That iron retards problematic flora needs some citation.
Bacteria will likely flourish more in an iron rich environment. But BV is different kinds of bacteria. At the end of the day, it’s a ratio. If the lactic acid strains become fewer than the pH changes to more alkaline. Sperm and blood increase the alkaline levels by diluting the acid environment of the vagina.
