The key information you provide is that your RPR was negative and your cerebrospinal fluid (CSF) (i.e., your spinal tap) was normal. This assumes, of course, that the CSF exam included all the standard tests, i.e. measurement of white blood cells, protein level, and a syphilis test on the fluid. If those were normal, you do not have neurosyphilis, or any other kind of active syphilis.
Personally, I would have recommended no syphilis testing once the RPR was negative; it is rare for neurosyphilis or any other complication of syphilis to occur with a negative RPR. Had the FTA-ABS not been done, the question of syphilis never would have come up. However, I am not blaming your doctor for doing the FTA-ABS; most physicians are trained that way.
Putting all this together, there is no reason for you to believe you have anything other than sarcoidosis. Certainly you do not have active syphilis, if you ever had it at all. (And the sexual exposures you describe were essentially zero risk for syphilis anyway.) The FTA-ABS is said to always indicate past syphilis, but there are exceptions. Some diseases sometimes cause false positive results, and sarcoidosis probably is one of them.
Bottom line: Forget syphilis. Continue to trust your own doc in management of your sarcoidosis.
Best wishes-- HHH, MD
Just an idea of my profile
29 year old white heterosexual male
Zero homosexual encounters in lifetime
Zero heterosexual anal sex
A few unprotected vaginal & oral sexual encounters between 1998 & 2003
One unprotected oral receptive encounter with prostitute 11 months ago