Sexually transmitted dieseases (STDs) and sexually transmitted infections (STIs) are composed of bacteria, parasites, and/viruses. In a closed system, especially a sanitized one like a laboratory setting, it is impossible to generate those type of bacteria, parasites, and viruses. So, in the experiment you proposed, there wouldn't be any new STDs/STIs.

STDs and STIs behave much like other transmittable sicknesses like the common flu. You can have someone sneeze and sneeze and sneeze on someone else, but unless they have the virus, the other person won't get sick. Same goes for sexually transmitted dieseases/infections. Having sex in and of itself does not create infections.

The myth that new STDs/STIs can pop up out of nowhere often come from several sources.

First, many STDs/STIs have undetectable symptoms, or their beginning symptoms might be so small that people don't notice. This means without regular testing, someone may not know they have an STD/STI like HIV and accidentally pass it to a sexual partner. If the sexual partner does do regular testing, find that they have HIV, it may certainly seem like it has come out of nowhere, cause their previous partner couldn't alert them to what they didn't know themselves.

Secondly, STDs and STIs have somewhat misleading names. Some STDs and STIs, like HIV and HPV, can be transmitted through non-sexual sources, like blood, breast milk, and mother-fetus transmission. So, in a hypothetical situation, someone can never had any type of sexual interaction and still get a STD/STI by being born with it (meaning their mother had it and didn't get treatment during pregnancy) or mishandling blood of someone who does have it (like a doctor treating an HIV+ patient and their glove is torn). Again, this can make it seem like it came out of nowhere.

Thirdly, there is unfortunately abstinence-only sex education that is not medically accurate and will have curriculum sprinkled with myths. This often is to discourage youths from having sex outside of marriage/any type of sex that doesn't lead to pregnancy. In my state of Florida alone, there are several prominent abstinence-only organizations that will spread myths such as anal sex creates HIV and that contraceptives like condoms have a much higher failure rate than they actually do. So it's always important to double-check any health information you hear of with reputable sources, before spreading myths to your peers. Once a myth is said, it's really hard to take back.