The per-intercourse level of HIV transmission for vaginal intercourse in otherwise healthy people is startlingly low.
Well under 0.3% for those not being treated with antiretrovirals (may be essentially zero for people with very low virus levels).
Other diseases can significantly increase this level.
(this is typically studied by studying couples where one has HIV, one does not, and they have regular intercourse. This can happen where you diagnose someone and can work out definitively when they were infected and then ask about intercourse between now and then)
(anal sex risks are rather higher).
Disrupting healthy skin balance might do all sorts of things, from roughening the skin and weakening it, to (perhaps more likely) the bacterial attempting to colonise the receptive partner and in the process causing inflammation/changes which more easily permit HIV entry.