No this would not change my opinion. Syphilis is just not a concern in the situation you described. EWH
I did want to note that my main concern was that I gave my girlfriend syphilis before I was on the doxy(which cured my syphil) and then her symptoms(anal lesion) showed up 9 months later.
So my probability should have read:
1 in 10000(I had syphilis to begin with) X.5(50% chance the antibiotics my girlfriend took did not cure the infection)X.33(2/3 infected are homo/bi sexual)=.0000165
I do not know if that would change your opinion.
Thank you very much for putting my mind at ease! Below are some interesting articles I found online that also relate to the rarity of syphilis. I'm sure you already know this but maybe it will help some people.
http://www.avert.org/std-statistics-america.htm
http://www.statehealthfacts.org/profileind.jsp?cat=2&sub=29&rgn=42
Thanks for your thoughtful post. My sense is that as you have researched and worked through your chances of having syphilis is that your estimates, if anything, are too high. You are correct that syphilis is a bit more common in SC and other southern states but it is still rather rare.
As for as your GF is concerned, I hope it will give you further peace to know that IBS can cause ulcers of the sort you describe and that azithromycin is active against many strains of syphilis.
Bottom line, this is not something to worry about. I hope you find this note reassuring. EWH