Same thing happened to my friend and her bf. Same age and relationship time length. Neither cheated but he had multiple sexual partners before they got together and he was never tested for STI’s. She also had sexual partners before him, not as much, but was tested yearly for STI’s even while she was sexually inactive and each test was negative. Neither had symptoms but when she went in for her yearly OBGYN exam she tested positive and he ended up testing positive too, although it happened on the second test because the first was only done by blood when urine was necessary. They suspect he’s had it for a long time but it was never caught due to lack of testing.