And where can they get sexually ill from if they don't change partners and don't get sick. But there are hidden infections that you can pick up even in the pool or drinking with an infected one from the same mug.
Actually, you can. Unfortunately, there are all sorts of low-quality condoms and lubricants - with them you can easily get thrush, or even chlamydia, as in fact, with neglect of personal and / or intimate hygiene.
You can catch a very dangerous parasite that will destroy your nerves, money, and free time. For some, such parasites drive crazy or even get rid of the world. These parasites are, of course, called children.
If we talk specifically about infection, then yes, there is some risk of contracting diseases with a long incubation period: HIV, hepatitis. There is also an option that not all infections were tested. Few couples are screened for herpes simplex virus, cytomegalovirus infection, or HPV before unprotected contacts. If we talk about standard STDs, it is almost impossible.
In addition to what has already been written, I will add about "clean after checking". The test may not reveal a disease that has appeared recently, for example, antibodies to HIV may not appear in the tests until six months after infection.
Well, yes, everyday ways of infection with the same syphilis are real, although rare. And your partner (or you) may not get syphilis in bed, but bring it to your bed.
I was also asked to answer, apparently because I am a doctor. But I am not very competent in this matter, I have a different specialty. However, I completely agree with the posted answers and there seems to be nothing to add.
The probability of infection is always there, the same thrush, a very annoying and unpleasant infection, is transmitted in public places, well, if one partner picks it up somewhere in the toilet, then the second will naturally become infected, chlamydia, are transmitted in the same way, in public places, be careful and visit specialists every six months 👌
Gonorrhea, syphilis, chancre, trichomoniasis is almost unrealistic. Almost - because although the everyday route of infection is very unlikely, but absolutely not excluded.
Something like ureaplasmosis is a less unequivocal answer: with a single examination it may not be detected, and such infections can also be attributed to opportunistic flora. That is, it is present in many, the clinic does not give, but under some conditions it can.
Finally, as Victor correctly wrote, there are many different flora in the genital tract, some of which are of course conditionally pathogenic. The same yeast bacteria. Well, for example, non-gonococcal urethritis in men, therefore, may well be with a healthy partner.
You can. Candidiasis can be caught from stress or from ordinary bathing in the river - and a number of other troubles. In addition, only when a new partner appears, some change in the microflora of certain parts of the body may occur - not because the partner is sick, but simply because he has his own "unusual" microflora.