High rate of sexually transmitted infections

Everyone has been going to the CDC for coronavirus data, but there are other diseases:

CDC estimates 1 in 5 people in the U.S. have a sexually transmitted infection ...

It is estimated there were:

Nearly 68 million STIs on any given day in 2018 (prevalent STIs).
26 million newly acquired STIs in 2018 (incident STIs).
Nearly one in two incident STIs were acquired by people aged 15 to 24 years old.
Nearly $16 billion in direct lifetime medical costs resulting from STIs acquired in 2018.

This is all pre-pandemic. We should have more recent data than 2018. Have STIs been going up or down?

The next thing that surprises me is how high these figures are. I would have guessed that about 80% of the population are leading lifestyles that do not expose them to STIs. That is, they are faithfully married, or not in sexual relationships.

That would seem to imply that most of those who are sexually active with more than one partner are getting infections. I find this hard to believe.