If monogamy is “unnatural”, then why do STDs exist?

I would like for the polyamorous people who behave as if preferring or being in a monogamous relationship is sacrilegious towards nature to answer this one. If all people were polyamorous or polygamous, the STD rate would be much higher than it already is. Are you sure that incurable STDs such as herpes and HIV weren’t created by nature to act as a deterrent towards such relationships?

Also before anyone leaves any crazy comments, I’m not judging polyamorous people as a whole. I’m referring to the ones with a superiority complex. The ones who put down monogamous people as if monogamy is some kind of crime.
If monogamy is “unnatural”, then why do STDs exist?
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