How does syphilis affect the male reproductive system?
By producing lesions called "chancres".
Although a chancre is a lesion full of the organism causing the disease (a spirochete, Treponema pallidum), it is important to know that almost all of the time, it is painless.
It doesn't have a direct effect on the male reproductive system, though. The only probable effect is once infected, and that male slept with a woman, the woman will contract syphilis as well. because the lesions are infective.
However, if the lesion is touched by anywhere else in the body(hands, mouth, etc) the lesions can form there. So in this example, I'm gonna post a picture of a syphilitic chancre in the mouth because I don't know yet if this site allows pictures of sexual organs for presenting the disease.