Why is it okay to have condomless sex with long-term partners (months, years) but not with short-term ones (days, weeks)?

We often hear that we shouldn't have sex with too many different people due to STDs, as if monogamy by itself protected us, but from my research that is not the case. We're always at risk. STDs are simply viruses, infections and diseases. As such, they can come to us in a multitude of ways apart from sex,... show more We often hear that we shouldn't have sex with too many different people due to STDs, as if monogamy by itself protected us, but from my research that is not the case.
We're always at risk. STDs are simply viruses, infections and diseases. As such, they can come to us in a multitude of ways apart from sex, even genetically and the worse is that we can have them without even knowing. This means that the only sure way to be safe without condom, not matter who you're with, is to have them tested for all deadly STDs before having sex with them. Well, this is what I've learned. Is it true? If it is, it would be disturbing because almost no one get their long-term partner tested.
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