From JAR-FCL 3 - FLIGHT CREW LICENSING (MEDICAL):
"(a) An applicant for or holder of a Class 1 medical certificate shall have no established medical history or clinical diagnosis of any sexually transmitted disease or other infection which is likely to interfere with the safe exercise of the privileges of the applicable licence(s).
(b) Particular attention (see Appendix 7 to this Subpart) shall be paid to a history of or clinical signs indicating:
(1) HIV positivity,
(2) Immune system impairment,
(3) Infectious hepatitis,
Now maybe I understood something wrong, but wouldn't the activities likely to transmit syphilis be a bit of a potential hazard in-flight? Regardless of whether or not the person in question actually has syphilis?