It can only spill if it’s not already present. Since it’s already present, the STD will have a larger “concentration gradient” to “flow” faster.

But, really, the STD rates will always be lower among the boring “square” population than the adventurous, promiscuous, one.

I’m sure this will be about as popular a soiled diaper here, but “promiscuity” and “adventurous” sex is mathematically welded with risk. Monogamy, and vanilla sex are essentially risk free (barring a botched blood transfusion or something along those lines)