In the 1990s I was staying with a guy in Miami for a while, and we had sex daily. I topped him wearing a condom and also did oral on each other. He told me he was clean.
But finally he confessed to me he had been lying - he actually had Hep C! You can imagine the shock that went through my system.
And at the time I didn't know the difference between the 3 types of hepatitis: A, B & C, I thought they were all highly contagious, even without sexual activity. I now know that Type C is the more difficult to transmit to another person, but I didn't know it then. I was seriously scared.
His excuse, a way of complimenting me and sorta reducing his guilt, was to tell me he was blinded by his desire for me, that I was so hot, taking him for rides on my motorcycle, dressed in my leathers. Yeah, right. Even 18 years ago I wasn't hot.
I immediately moved out of his place, feeling totally betrayed, and worrying for my health. Later tests showed I was not infected, and remain Hep-free to this day. My understanding is that hepatitis can take a long time to appear, so for years I fretted, but at this distance I'm long out of risk. Yet my blood panel always tests for all types of hepatitis, plus HIV, STDs and other infectious diseases, all negative so far.
Moral: men lie when they want to have sex. Men can have diseases that have no outward symptoms you can detect yourself. Condoms are very effective for HIV (I lived with an HIV+ partner for several years, and was the bottom, I'm still negative). But condoms do not protect against all STDs, because some diseases do not come solely from the urethra.
Treat every new partner like he's HIV poz, and could also have some kind of STD. Regardless of what he says himself, he might not even be aware that he's got something. He could pass a lie detector test, honestly believing he's clean, but still be infectious. That's what having "safe sex" means.