If a man has venereal disease like herpes, how would it affect his status change through marriage?

The marriage-based adjustment of the status application process includes a medical examination and determination about whether an applicant may have a "communicable disease of public health significance" per INA 212(a)(1)(A)(i). Some sexually transmitted diseases/venereal diseases can render an applicant ineligible for immigration benefits, such as gonorrhea and syphilis (at an infectious stage). Of course, health-related grounds for inadmissibility are only some of the details that determine immigration-related eligibilities. It would be wise for the man you mentioned and his spouse to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options, and strategies to attain the couple's goals, and who then could offer legal representation in the often complex application process.